Could mu computer's PCI-E slot be causing video cards to fry?

So in the pass 2 months I've had 2 cards. 8800GT nvidia and ATI radeon 5670HD DDR5 2.5gig. My 8800GT ran like 2 times and it does a green tear in the screen on startup and never worked again , So i saved up and got this new 5670 ATI radeon last night. It worked good , But it did the SAME EXACT green tear in the SAME place on the screen on startup and now it's not working. Could my PCI-E slot be causing this? Or just odd ods that the cards where both bad?|||One thing is for sure, I sure wouldn't put any other cards into that system. Could be the motherboard or the PSU...