Have you checked the PSU? It sounds like it's failing.

Try recapping it if you haven't already (if you have, then I guess it's time to look for another one). Uniserver can probably do it for you if you can't/don't feel confident enough to do it yourself.

If that doesn't work, then maybe there's some cap goo on the logic board somewhere that's still causing problems. Try giving it a thorough cleaning.

c