Marty - Adamski & Chairman have some good points there. It's odd how sometimes just unplugging / removing something & then reconnecting / plugging it back in can fix a niggly problem. RAM is the worst offender for this, but reseating the video card can be worthwhile as well. I wouldn't fiddle with the CPU unless the problem comes back and / or is persistent. The "sniff test" can be quite a good one - that "brown smell" can be quite a useful clue. The only problem with the sniffing method I've found is a) one sometimes needs to dismount the motherboard to get one's nose close enough b) if one's sense of smell isn't great, the amount of "brown" can sometimes be too small to detect and c) It's sometimes hard to really localise where the "brown" is coming from. But all that said, it's still a useful test.

I've purchased a PSU tester (available on TM fairly cheaply) that I use to test all the cables coming from the PSU. Such testers are great for picking up obvious faults, but most of them aren't able to simulate any sort of "load", so it's possible for them to report a "clean" PSU but a fault is still there that only shows up when actual real-world usage takes place. A common enough fault that the PSU testers often DO pick up is when one of the main 5v or 12v "rails" has failed. Of course, not all PSUs have both -5v and +5v (so it pays to check for that), and many PSU testers aren't able to show which rail in a multi-12v rail PSU has failed (some PSUs have 2, 3 or 4 12v rails - but many only have one: I've not struck a case of a PSU that only had 1 failed 12v rail when there were 2, but I note that my tester, like many, only has a single light to indicate 12v power is there or not, so I don't know what would happen if 1 out of 2 rails was faulty - might just show no light?).
Unfortunately, if all the tests reveal nothing, there's still the possibility of the motherboard just being "weird": lost count the number of motherboard over the years that seem fine, look fine, test fine, but just do odd things, usually getting progressively worse until they just won't allow boot at all.

I hate discovering those types after wasting many hours thinking "what if I just try this next thing...?"
