Dell latitude e6510 won't boot won't start diagnostics

My latitude started acting like it was "over heating" about a month ago, meaning, it would not turn on correctly after prolonged use. I re did all of the silver paste the first time and cleaned it out and that seemed to do the trick. It has happened once or twice more and always started working again after a night of resting.

It has now stopped working again, and I believe I must have a problem that I need to fix.

Laptop:

• Dell latitude e6510

Things I've tried:

• Reseating processor
• New silver paste
• Memory sticks one at a time
• New battery
• Different boot hard drive
• Reset bios to default
• Booting to diagnostics(does the same black screen, no cursor)
• Booting normal

It is not automatically showing me any diagnostic lights at all.

I imagine at this point it could be a main board issue, or I need a new processor, but if there is something else I can try first, I'd like to.

Thanks to anyone with a clue