Graphics Card Problems?

Just recently I bought a used Radeon HD 6870 video card from Fry's and a new power supply. I had troubles installing both the power supply and the card into my computer so I sent it in to a local computer repair store. I got it back the next day. Their report said when they were installing the drivers the system randomly froze but they got it to work. Afterwards they tested Portal 2, Spore, and Left 4 Dead 2. They worked. However, before I got this new card I had an Intel Media Processor for my "video card". The Intel card would play Portal, Left 4 Dead, and Spore, but it would not run Mass Effect 1 or 2, Magicka, or Splinter Cell Conviction. The new card that I have runs Portal, L4D, and Spore perfectly. But when I try to play the other games listed above I have problems. The system will freeze randomly in the title screen of each game. I restart my system and retry it a few times. After a few more tries the game doesn't freeze, and the game runs like a dream. However, the next time I try to run it, it freezes again, and I have to restart my system a couple of times to get the game running again. What might be the problem?