Old Nvidia 9500 working better than newer Radeon 4870...

Hey guys, I apologize in advance for the long post...

I just registered but have browsed the forums from time to time, they have always been a great help. I am having an issue that i hope someone might be able to help me narrow down to what component might be causing it. I am posting in the graphics forum because that is where i am having the problem while gaming. Going to try to give as much info as a can so here goes...

I have 2 pc's one at work and one at home that are nearly identical. Both i put together fairly recently using AMD am2 5000 cpu's. One of them has a 5 year old(at least) Nvidia 9500gt video card and the other has a newer ATI radeon hd 4870. Now according to the video card comparison site the radeon card is much better, which is should be.

http://www.gpureview.com/show_cards.php?card1=574&card2...

Now, for some reason while playing on the machine with the older 9500 i get consistent 40fps but on the Radeon machine my fps is bouncing all over and usually sits at 23fps. (i am using age of conan as a reference point, both machines set with the graphics options low) Over the last 2 days i re-formated both computers and all that is on them at the moment is a fresh install of windows XP, the video cards driver software(nvidia control panel, ati catalyst), 1 game for testing and 1 benchmark program. I tested both last night and the issue is still there although i am getting a bit higher fps on the radeon machine BUT, now every 10 seconds or so the game will freeze for a split second, really noticable while running.

So, the only thing i did noticed last night before bed is that the radeon machine only has a 380w power supply in it which i started thinking was the problem, but today i ran the antec power supply program
and if i am doing everything correctly, it is saying i only need a 347w supply. I have a spare antec 550w supply im going to try tonight but I thought i'd post this up and see if anyone has some suggestions.

One last thing, i ran 3dmark03 on both machines and the radeon shows results 3x higher, do benchmarks put a good load on the system?

Thanks for your time,

Blake