If I understand correctly, you supplied the card's power using the cables you got with the video card right? You shouldn't do that - you should...
I googled a bit and it seems the 8400gs requires a 300 watt PSU at a minnimum for stable system operation. Seeing as how you have a quad core...
I had an almost exactly the same problem with my old 8800GTX - the frame rate would drop to about 5-10 FPS for about a minute or two then return...
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