The trouble with these machines is that it takes a REAL COMMITMENT to troubleshoot them, even when you've got a parts machine to play with, because disassembly/ reassembly is not a trivial matter. The upshot is that almost nobody does it, and since almost nobody does it, there is scant experience to draw upon.

Mind you, the fact that it has been know to boot or to boot partially may be a good sign. Have you tried booting from an external drive/ different RAM?