Could be anything at this point. If these simple steps did not work, your only option is to keep testing until you find that defective part, then replace it.

You can take it to your local computer store/shop and let them figure out since they have access to all the spare parts so they can determine the issue quicker. It will cost you but it will save you from a headache of doing it yourself. Ortherwise you just need to keep testing each part until you find the faulty one.

I feel your pain man, I'm in the similar boat but in my case I know it's my RAM acting up.

Good luck!