Upgraded Dell C521 locking up

My son and I play Call of Duty World at War Zombie Level with 2 of my brother-in-laws. M sons computer is a Dell C521 with an AMD Sempron 3400+ CPU. I had to install a better video card in his PC to get the game to play.

Orig Config. of Dell C521
CPU: AMD Sempron 3400
Graphics Card : Galaxy Nvidia GeForce 8400 GS (Low Profile)
RAM:2 GB
OS: Windows XP
Power Supply: 280 Watts
BIOS version: 1.1.11

Since the CPU was just barely able to play the game it played sluggish and really bogged down in the higher levels to where he was getting a 300+ ping on our home Lan and would eventually drop out of the game.

After looking around on the web I saw that some people were able to upgrade the C512 with an AMD Athlon X2 Dual Core chip. I got an AMD Athlon 5600+ CPU and replaced the Sempron with it.

I bought Arctic Silver 5 Thermal compound and the special cleaner. I cleaned the 5600+ then applied the Thermal compound to the top of the CPU as per the instructions. I put the original heatsink back on and re-started the computer. It came up great and much faster. I tried playing COD WaW again but it would lock up after a few minutes.

The computer works great outside the game. The only time I would see the lockup was when playing COD WaW. Everything would freeze with a buzzing sound coming from the speakers and I would have to reset via the power button

I became concerned that because of the higher CPU usage and graphics the CPU was overheating. I downloaded a program called core temp and monitored the temperature of both CPUs. They would stay in the 30's with normal use but when playing COD WaW they would move up to the high 40's low 50's. The highest I think I ever saw them read was maybe 58 for a second.

I've tried adding updated drivers for the new CPU:
AMD Processor Driver Version 1.3.2.0053 for Windows XP and Windows Server 2003 (x86 and x64)
AMD Dual-Core Optimizer Version 1.1.4

I've tried playing the game with the case open and a fan blowing directly in the case. The fan seemed to help but the game ran sluggish (and I died on level 4)

So, is this a heat issue, a driver issue, a power issue or something else?

Because the C521 is a low-profile case I'm kinda limited on what I can do. The 5600+ is a 65w CPU. I could downgrade to a AMD Athlon X2 5050e which is a 45W chip and might produce less heat.

Any suggestions?

Thanks,
LJW