450 Workstation lockup

I have a precision 450 workstation service tag 3GCK821. This system has been locking up frequently. I have placed two service calls and it doesn't seem to solve the problem. The first time, it looked like the hard drive was bad. When booting the system, the system would show windows xp boot error about system files not found. The tech sent a new hard drive to replace. That seems to resolve the problem. After awhile, the system started to show the same problem and random lockup started to occur when the system is on. When it freeze, the mouse, keyboard and system will not respond to anything. Cold reboot is the only way to restart. Re-installed OS and it would work for a few days and the same problem comes back. Called support. We thought it might be the memory or board. The memory and board was replaced. The tech also had me run the dell diagnostic. I ran it for 24 hours. No error was shown. After replacing the board and memory, it seems like it solved the problem. However, lately it has started to lockup and windows errors are reappearing. I'm looking more at the CPU as the problem. That is the only thing that has not been replaced. Is there a solution to this problem?