The flupping could be for a lot of reasons, perhaps multiple coincidental reasons, so it's going to be difficult to provide advice. It could be the voltage adjusted too high for some reason and it's crowbarring and needs to be adjusted down. It could be bad capacitors. Or it could be a blown resistor or diode. Or a bad optoisolator. Or a bad SCR. Or a cracked solder joint or bad connection on the logic board connector...
Personally, I'd start by establishing a known baseline. First, do a visual inpection, look for broken or burned components or connector pins. Resolder all the connector joints, and verify continuity of the cables (in this case, especially the logic board cable).
If nothing results from this, then check diodes and resistors. It takes a little while, but not too bad. You're just looking for something obviously wrong (i.e. shorted, or super high resistance).
If still no improvement, and you haven't replaced the capacitors yet, then I'd recommend replacing all analog board capacitors next -- again, to establish a known baseline. They're often requiring replacing by now anyway, so it's good for future-proofing as well. Heck, you might even want to do this before doing the diode/resistor verifications.
If still no improvement, then we start diving in to the schematics or other troubleshooting docs (like Dead Mac Scrolls, which was written when these machines were still not very old) and looking at power side components - which do commonly fail, like the 4N35 at Q3.
