Hmm, now you say you had this problem with an older vidcard, and it happens with this one too eh? so either you somehow had 2 bummed vidcards, or your mobo's messed or your psu. If you can, test the vidcard in another computer that you know runs fine like a buds. Say you'll pay him like $5 to test it. I bet if he has any brains he won't argue, $5 to do a 10 min test or so. Good deal.
So yes, test the vidcard to be sure it works fine.. I just wanna make sure cause I hear there's a lot of bad AGP 6600GT's out there. XFX with bad vga coolers (they apparently fixed this with a newer revision), msi with defective ram.. there's probably more examples out there too.
If this checks out then yeah, I bet it would be a bad psu or mobo.