So the problem started recently. That means it is either: a cap going bad. oxidized contacts. dust clogging the fans. PSU is going bad. something obscure.
Do the easy thing first. Clean your case, reseat all cards and memory modules and check all caps while doing so. Any of them deformed? The 'head' going up? Strange stuff around its feet? Congratulation, you need new hardware. If you don't find a bad cap and the problem persists, get a new PSU. A good one. Not big - most PSUs are oversized, but good quality. Anandtech has something about psu's, so does tomshardware (most of their tests are rubbish, but their psu tests are ok). If the problem goes away, congratulation! If not, well, then report back ;)
