is it actually going through the whole boot sequence or is it just continuously flashing the led and just looking like it's going all the way through?
I was sent one that continuously flashed the led and it turned out to be the solder throughs above U8 ram.

try what Neno has suggested and see what happens.

Does it still have the old style I.C, sockets?
if it does the pins on these sockets tend to disintegrate when there has been a battery leak.