Mac Pro 2008 El Capitan Kernel Loop

You could try one of the utilities that will display the various temps reported in your MacPro.
I use smcFanControl, but there's also Macs Fan Control - and a few others that I can't think of right now.

That can show you if the temps are spiking when you get your reboot. If the temps don't seem to change noticeably, and not really connected to the reboot, then it may simply be an issue with the power supply.
I will check those out. I did use TechTool pro and it told me that the fans all operated at a proper speed. I did not notice if TechTool pro checked the temperature or not. Thanks again for the help!
Over the past couple of weeks, I have figured out that the computer runs for hours as long as I don’t go to the Internet. I worked in iTunes today, worked in Photoshop, did some work in Illustrator. Once I went to Safari, the Mac locked up, screen went blank, and went into reboot. Any ideas on what would cause this? Is my ethernet card a good bet to be the culprit?
Does that lock up/reboot happen with another browser (Firefox would be a good choice for a test)?

Do you get lockups on a different user account? If you have only one user, this would be a good reason to create a new user, then log out of your present account, and try your internet in a different user account. Be sure to also try the other ethernet port on the back of your MacPro.

Did you ever try out a utility that will display the temps that are reported in your MacPro? Could be that when you go online, the temps spike for some reason. Worth checking on 10-year-old Mac hardware.
I deleted the 5 .plist files for Safari and the one for Firefox. After a restart that I initiated, I launched Safari and it crashed. My next move was to launch Firefox but haven’t got around to that yet. I may do a new install of Safari to see if maybe the app was corrupted somehow. What led me to the Ethernet card is that I did use a different hard drive with another account and again the Mac shut down once I was on the internet. Both Safari and Firefox.
Firefox has been running for 10 minutes so far and no lock ups - that's an improvement!
DeltaMac - I did run one of the the Fans apps you mentioned before. What app will help me monitor the temps? MacToolsPro checked the sensors and all were nominal.
Well, I left the room for 5 minutes and came back and Firefox had locked up. The monitor was black across the bottom 2/3 and white across the top. I shook the mouse and the screen came back for a moment but then went black and white again. That was short-lived.
HWMonitor is pretty slick. I'll install it when I get home and see if it can tell me what's going on. I like the fact it's on the desktop all the time and you don't have to switch over to Widgets to view. Thanks, DM
Power Supply Consumption wavers between 138W and 200W, especially when I launched
Firefox. My MacPro 2012 at work stands steady around 100W.
Power Supply Voltage holds around 12.125V.
Not sure what I should be looking for. I'll see if the user manual has a table of nominal settings.
Hey DeltaMac, I’ve been reading about possibly speeding up the fan speed towards the rear of the Mac to cool down the video card. From what I’ve read it’s helped with the screen freezing and crashing. Any thoughts on this tactic?
Assuming that the freezes and crashes are caused by the video card (and not something else), then, sure do whatever you like with fan speeds. Your video card is an NVIDIA GeForce 8800 GT, which has its own fan. You could pull the card out, and take a look at the fan to make sure that it is clean, take it apart to clean if it looks nasty to you.
I did as much cleaning as I could and did pull the video card, got the outer shell off, and dusted pretty well around the fan and heat sinks with canned air. I removed the fans, both front and back, and give them a good cleaning. I guess my concern with the readings I got from HW Sensors wasn’t as much the temperature around the power supply as it was the consumption that was peaking around 200W. I wish I could find a place that had a full listing of the normal operating temperatures for the system. Or maybe tomorrow I can get a screenshot of the readings and you could give me some feedback? I appreciate all of your input so far.
I think that the temps that you see are simply normal for your use. And, the power draw for the power supply likely just tells you that the 2008 MacPro is a relatively inefficient power user. It would be difficult to compare pure numbers for power from your older MacPro to the newer one, which tends to have lower power use on average.
What you want to know is what is happening when your system misbehaves.
Have you checked your RAM?

Long ago, I had on a previous machine one of two chips go bad. This meant frequent kernel panics particularly when using applications that can use suddenly lots of memory like browsers.

The bad news is, back then, TechTools Pro consistently claimed the RAM was fine. A few tech savy recommended I not trust that. I tried the now old but still good "memtest" and it showed immediate failure. TT may be better at detection now.

If that is the case, your RAM may be under warranty--mine was.

Thank you Doctor X. Definitely something I will take a look at. I’m running into dead ends and might just have to spring for having it analyzed at at MAC service center
This is the HWMonitor just before carshing yesterday. I had raised fan speeds to see what, if anything, they may do if the back end of the Mac was overheating.
Yeah... What a mess!
I suspect that those "264˚F" are probably bogus, as that relates to about 129˚C, well above overheating point, with your fans showing barely above idle. The 264 value might be the highest number that the software will show, but I have no way of proving that, other than thinking that if that temp would be real, the CPU would trigger a shutdown.

I would suggest that you should adjust so the fans run at max speed for a few minutes, particularly if you have discovered any situation that "seems" to result in a crash. Watch for the same item's temperatures then!
I turned all fans to max for maybe 5 minutes and did not see much change before and after.
I should mention that I only have 2 sticks of RAM so would those super high readings be coming from the empty slots? Don't know. Can a dual processor run with just one processor? Can I unplug the one?
Here is what Macs Fan Control screenshot shows after I reset all the fans back down to normal.
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