how to know if a video card is failing

hi, can anyone help tell me what benchmark or software to use to test my radeon 9800 if it is failing. the stupid store that i bought my radeon doesnt wanna replace it because they said its working fine. i still have the warranty but i have only 1month left on it. and i cant prove to them it has problems because i put it in my friends PC although it doest crash, it lags big time. and i put his 9800XT on my system and i played 4 straight games without crashing. and they kept saying that they tested the card but found no problem. i even used my old fx5200 and that didnt crash too. i just have to lower some bench detail low and shadows off. enyweiz i hope anyone can help me. thanks in advance. i paid $400 for that card 8 months ago and what they want to do now is to replace it with either a radeon 9250 or geforce 5500 because they said they dont have 9800pro on stock anymore

my specs:
amd xp 3200+
1 gig ram
MSI motherboard
ati radeon 9800pro 256mb
430watt power supply

what do you mean by crashing?
pc freezes? pc reboots?

pc freezes after 1st quarter playing 5mins a quarter. i have to manually reboot the pc after it freezes and crash.

sounds like a temperature issue. try opening the PC and putting a big fan on it to make sure it's really cold. then try again.