For dual processor systems, there is a tight tolerance for variance between the two processors. For Xeons in particular, you need to ensure both processors have the same stepping number. If both processors have the same stepping number, you may (as someone suggested earlier) want to try running with a single processor and a terminator. If you have the same problem with a single processor, try switching the processors.

I recently purchased a Dell 6100 with 4 processors. The guy I bought it from had installed 2 512k cache processors, and 2 1024 cache processors. I was unaware of this at first, so I booted it up. Needless to say, the system halted due to a cache conflict. I quickly removed 2 processors, and it booted up flawlessly.