Weird locking up issues

Alright so I've been having some strange issues ever since I got a new graphics card and PSU put into my computer. Basically, during a lot of games I get stuttering when I never did before (two examples off the top of my head Left 4 Dead and Project Reality for BF2). Also, certain games I get fairly frequent lock ups >>> VPU recover. I never used to have these problems with my old Nvidia 7900GS. I now have a Radeon 4650 with a gig of memory. My old card had like 256. I know the 4650 isn't exactly hot stuff but it's better than a 7900! I DO get better frame rates now of course, but these frequent lock ups and stuttering are driving me crazy. I'm almost 100% sure it's not an internet issue because when the stuttering happens my ping does not go up at all and I have the exact same internet as before. I did changed router, (got a much better one), but the problems were already happening before I changed.