The obvious thing would be to replace the logic board's capacitors. Washing a board (I do this too) can enable a machine to boot by removing electrolytic residue, but it cannot resolve the underlying electrical problems caused by original parts that just no longer work.

The next thing would be to experiment with different RAM, and in the first instance, I think I would try just one bank, as the problem could be something like a bad memory slot. I also find my SE/30s to be very sensitive about RAM.