8800 gts 512 fails then works

Hello,
I have a 8800gts 512 which has worked faultlessly for about two years, I am running xp pro and no overclocking of the gpu. The other while using the machine the video went crazy and had lines across screen. The machine reverted to 800x600 and had a error code 1 on the card in hardware manager. I took the card out after trying to change drivers and everything . I then thought I would try it in my second machine , it started up with the lines still present in bios and start-up. The machine said it had to restart and the card worked again with no imput from me . I was convinced it had fried, but I then put it back into the main machine and it worked perfectly and no sign of the trouble for the last couple days. Any ideas what had gone wrong? is the card on the way out ?or was it just driver related .