Sounds more like a system board problem

I thing I had the same problem sometime back

Replaced the system board

 

I was kind of beginning to think the same thing. Do to me switching processors and having the same error codes. I have never swapped out motherboards on a laptop. How hard is it too do?

Goto the following link for the D620 service manual

http://support.dell.com/support/edocs/systems/latd620/en/SM/index.htm

 

Good luck