I have another suggestion. This one is odd. Remove the outer case, point a fan at the computer (like a summertime table fan), and see if the problem repeats.
I suspect a hardware issue, like the power supply or overheating.
The first thing to do while you have the case off is check, when the power is on, does the CPU fan spin? Those things fail too, and soon thereafter, the CPU will get flaky and very soon thereafter, it can burn out.
Yup! I have one machine that's known for being....a bit rough on em, due to a couple of design flaws and my stubborn insistance on using it for EQ anyway. Long and the short of it is, first time it failed it took me over 6 months to diagnose it, bc it failed very slowly and only in EQ at first, then after a long time, in Diablo 2 also, but with a known iffy cd drive, I still never looked once at anything but buggy coding in EQ and "the bloody cd hanging again" in D2. Finally had to lock up in the patcher when it died for good before I pinned it down.
Next fan was a cheap POS that lasted about a month but had the grace to die cleanly. I stupidly exchanged it for another from the same shop and even same lot, just glad they didn't stick me for more money. BIG mistake! That one also died after about 6 weeks, and that time the machine had been idling for about 3 days with the monitor off...while I sat furiously disgusted at EQ's constant crashes yet again and wondering why it was still happening.
Turned it on to find it froze at WinME boot screen, no apparent cause....hit reset and it locked immediately past vid card bios info, soon as it started counting ram. I checked it out and of course found the fan sitting there defiantly still. No clue to this day how long it actually sat like that, but 2 years or so later the machine still runs, and shockingly enough, is still pretty reliable.
However, no one should take this to mean they should ever let a machine run without proper cooling...it'll toast em real quick. I just got really lucky and have a machine that's sturdy as hell and doesn't run TOO hot. In the end I also now keep a swamp cooler on it 24/7 bc the requirements for EQ have gone up and keep pushing the machine harder and harder, while the design flaws that make it prone to heat crashes just can't be fixed fully.
I've also replaced a couple power supplies on each machine, but none gave the symptoms in the original post. They've all gotten loud and rattly over time, and then just bogged down to the point the cooling fan is moving slowly or stalls totally. When whacking on them no longer quietens them or brings the fan back up to speed, it's time to replace.