Try to dismantle all of the socketed chips and clean the chip pins with isopropanol alcohol and reseat (make sure you put them back the same way as they were, having the little orientation notch in mind). Clean the RAM pins too, and maybe the RAM socket pins with an old toothbrush + alcohol again.

Look for any signs of corrosions/leakage near the PRAM battery. Also measure the axial capacitors with an ESR+capacitance meter if you have one (desolder the positive pin first for a guaranteed OK measure, the ground pin can be hard to desolder).

You should also check if the PSU voltages are in an acceptable range, be sure to measure this while the unit is on and connected to the logic board -- the PSUs needs certain load to have a stable output voltage.