USB dies after a while

Every once in a while, I loose all usb support (keybord and mouse is all i've noticed so far). It seems looks like this usually happens when programs put a little bit too much pressure on the prossesor, or when I try to switch to another window. Maybe something with iTunes (it's always open) since MP3 decoding (so that I can hear it) takes a lot of juice, or maybe something with my computer going to sleep (I dont think I have this problem untill I sleep my computer). Anyone elese have this problem?
I have the same problem in Mac OS 9. I do not have OS X installed so I do not know if the problem persists in this operating system. I have a tired old iMac. The original. The problem seems to happen eratically. I sometimes when I quit an application all of a sudden my keyboard stops working! Actually funny thing is that my mouse keeps working! All I usually have to do is just unplug the keyboard from the usb and then plug it back in. but it seems this problem is happening more often and today the mouse went out too I had to restart the computer (with a paper clip no less) for the usb to kick in again. This is quite annoying and I have no idea what the problem could be!
In your case, thats a crash. It happens all the time in Classic. I just loosing my usb while the rest of the system still works (iTuns still plays, I still get IM's, etc).
Update: this doesnt only happen after I come back from sleep mode.