G4 Mac Slowdown


I have been having an intermittent issue with my G4 recently.
Basically, this is what happens. I turn on the system and everything loads properly, no errors. I can open and use any application on the system. However, it seems that if I let it sit idle and the screen saver kicks in, the mouse movement will be very erratic and unresponsive.
I have a strong PC background, so I opened up the case and checked for any heat problems. Everything seemed pretty cool. I also noticed that if the dashboard is called up, the mouse won't move at all.
Another odd thing I've noticed is there's a kind of crackling, hissing sound coming from the back of the system (power supply maybe?). The sound will stop or change when things are happening such as mouse clicks, opening programs, scrolling up and down windows, etc.

Here are the system specs:
OS X 10.4.1
G4 400MHz Graphite
2 x 256MB PC100 SDRAM
40GB 7200rpm IDE Seagate HDD
Nu-Technologies DVD Writer
Stock Zip drive
Stock ATI VGA Card

Thanks for any help and suggestions!
What kind of mouse is that? Might not be the problem, but I've had similar issues myself since 10.4 with a wireless Logitech mouse. The Apple USB mouse works fine.
The mouse i was using was an AOpen-branded (really a Logitech) USB optical mouse. It was a little bit older, so I tried switching to a different type of USB mouse. Since switching the mouse I haven't had any problems at all!