Lag with an ATI Radeon HD 3850 AGP, Driver issues?

Just upgraded an old AGP rig on XP with an 3ghz pentium 4 and 2gb ram from an 7800 GS OC to a 3850. It got up and running OK and seems to work normally however it cannot run any game normally and begins to stutter after a few minutes of gaming on most games and gets around 30 fps until it begins to worsen (apart from BF2 which is just unplayable). After that it will hardly run things like youtube without stuttering. I checked the temps and it stays within 40C throughout so that is not a cause. The only thing I can think of is the drivers, Which I have had trouble with from the start. It seems ATI do not support this card and the drivers on the disk were out dated and also did not work (failed with driver signing at the end of installing ATI catalyst and will not run CCC) and had to download and install manually the latest hotfix from ATI atleast, I think that's what i did. If somebody could direct me to make sure that would be a great help to me.

It's not just a bad computer either, The rig could handle many games before (such as Battlefield 2 and source games)but since I upgraded (changed the hard drive and the Graphics card) The games have become unplayable.