My old PSU 300Watt, started to play up in this way. When attempting to start the system the PSU would shut off after a few seconds(crow bar protection?). The only way to restart was to remove the mains input plug, wait a few seconds and then replace it. This would take 1-20 attempts before the computer started properly.
I assumed the problem was an under powered PSU so got a Q-tec 550watt PSU (it was cheap).
The system has been fine for months, but has started with the same problem again. If I disconnect the hard drives during the initial boot sequence, and then reconect them the system will complete the boot sequence without any trouble.
I did recently upgrade to an XP2100+ although it was fine for a while.
Can the efficiency of a PSU reduce over time?
Abit KG-7 Raid
XP2100+ (Flower Cooler)
256M 2700 Ram
2 x IBM Deskstar 20G
2 x CDROMs
Acatel USB DSL modem
yes it can, due to many factors (erosion of wires - mostly in cheaper systems, heat-factors on components, etc, etc). Your 550W PSU does have 2 fans right ? And one thing I learnt the hard-way (my system used to shut-off at least 3 times a day) is DON'T skimp on the PSU, get a brand-name one... I mean, common, an Enermax really isn't that much more expensive, besides, an Enermax 420W will most likely be able to beat a no-name 550W PSU any day.
Have you ever stuck your finger in a Vantec Tornado? That's 5700rpm right there - ouch. Today's public service announcement: don't play with plugged-in fans - especially Tornados..heh.