[H] Odd CPU issue

[Thane Sherrington]

I have an HP machine that won't boot with its CPU in it (boots to 
three long beeps and then one long continuous beep.)  It has a 
ADA4200IAA5CU in it
http://www.cpu-world.com/CPUs/K8/AMD-Athlon%2064%20X2%204200+%20-%20ADA4200IAA5CU%20%28ADA4200CUBOX%29.html
When I put in another CPU ADA5600IAA6CZ
http://www.cpu-world.com/CPUs/K8/AMD-Athlon%2064%20X2%205600+%20-%20ADA5600IAA6CZ%20%28ADA5600CZBOX%29.html

It boots fine.  So one would assume, bad CPU.  But when I move this 
CPU to a test motherboard, the machine boots fine.

Any ideas?

T