Creeping GUI death

Question

Folks:
Recently, I've been experiencing a frustrating increase in the number of GUI crashes. Now up to at least one a day.
The first symptom is graphic trash showing up on the currently active window. Pretty soon trash shows up all over: on the menu bar and on --or around-- any window that I bring to the front. Soon much of the screen is trashed. Moved windows leave ghosts. Switching between apps --including the Finder-- becomes progressively slower. Within a few minutes the system is unusable. Rebooting is the only option.
Restarting the Finder has no benefit.
In the latest incident, as soon as graphic trash was visible, I started saving modified docs and quitting apps. As I reached nothing but the Finder running... the graphic trash (mostly?) disappeared and responsiveness returned. This is just one data point. I'm not sure quitting apps completely restored normal operation, but it certainly looked like the right direction.
The latest incident happened after I re-installed 10.5 from DVD and brought the system completely up to date using Software Update. (I also repaired disk permissions.)
What was I doing when these incidents occur? Just working, using well-known apps from generally well-regarded vendors. (Maybe I switch from app to app to Finder to app more than some people.) I haven't added any apps recently, just updated apps as updates become available. My application mix and load haven't changed significantly recently, either. My system is pretty clean. I use very few third-party apps. Full boat of system memory and plenty of free disk space.
So, what's going on? The slow-down sounds like a memory leak, but I'm not sure what the graphic trash means. Well, I guess that's some code writing to Graphic memory, but I don't know what that symptom means.
More precise terminology for what I'm observing?
Theories? Fixes? Places to look? Instrumentation?
TIA,
hen3ry

Answer 1

baltwo:
Thanks for your suggestions. None of them applied, either because the iMac model was different, or the symptoms didn't match.
I don't recall getting an Apple Hardware Test for this machine. I looked in my disti media and didn't find anything. Are such tests downloadable from somewhere?
After I posted, I experienced a plain old freeze, which was really a different symptom. I thought about the symptoms for a long time and eventually it popped into my head to check all the cables connected to the iMac. (With the system shut down!) Sure enough, the power cable was not firmly seated. Nothing happened after I re-inserted it and booted up, but I haven't had any problems of any kind since then, about a week. So I'm declaring this fixed.
How is it possible that a loose AC power connection could be good enough to support normal operation except at random times, and then with symptoms different from just shutting down? In decades of experience with computers, I've never seen this before, but I've definitely seen cases in which marginal power supplies caused erratic, mysterious behavior in electronic circuits.
Speculating: I think Apple designs their disk drivers with a very strong goal to continue working as long as possible in the face of hardware degradation. If the power supply hardware and supervisory s/w are designed with the same goal that would explain what I observed.
Solved!
hen3ry