I have been having weird problems with my old XFX 7900GS (600M). I heard that XFX makes reliable cards but this one has let me down after a year and a half only.
The problem is that the computer restarts automatically when the card is being used. Sometimes after an hour after start-up, but recently only a minute or 2 after start-up. Removing the card gets rid of the problem. Is there a problem with the card or any other device???
Instead , im using the onboard video adapter for the time being. My setup is a bit old but still capable of easily running Vista:
1. Intel Core 2 Duo E6300 (No Overclocking)
2. Kingston DDR2 2.5GB RAM @667Mhz
3. Intel DG965RY MB
4.XFX GF 7900 GS (OC to 600M)
Ok, i think it's your power supply and here's my reasoning:
you said that you have a 350W power supply with 25A on the 12V rail
Either this is a typo or your adding together two or more 12V rails into one which is not always accurate. In this case it's just plain impossible (assuming i did my math correctly). Assuming a 75% efficient power supply, which is generous since I would bet that yours it is closer to 65%:
350W*0.75-2=262.5W of available power
Assuming that you could use all of that power at 12V (which you can't) that would give you:
262.5W/12V=21.875A total amps to spread across the 12V rails.
I seriously doubt your getting the required 20A across your 12V rails, you were probably running your PSU at near 100% load for over a year and a half and i think it just can't keep up anymore. This recently happened to me, i had an old 400W power supply and eventually after several years it couldn't keep up with the 6600GT i had in that system and would cause the whole system to just lock up.
That's true, guess i just got carried away with the math... However I was under the impression that this was only true of very high quality manufacturers such as PC Power and Cooling? or was i mistaken? I guess the best way to know is to look at the label... So in retrospect i suppose it is possible to get 25A on a 12V rail from a 350W power supply but that still doesn't mean you can always add the amps together fully from seperate rails.
Regardless it still sounds overwhelmingly likely to me that this is a power supply problem do you agree? However It could still be a bad graphics card (or even a bad motherboard i suppose) so try to verify the problem before spending more money.