Did you replace the large capacitor in the corner that holds up the 12V via to ground? looks like 0.8 to 1.0 uF would work there. That would be my first choice for items to replace.
Does the "12V" supply seem to hold up or drop when heat is applied, either to the cap or anywhere else on the board? is the 12V stable or does it get affected when the circuit is misbehaving.
Also the same question for the +/-5V supplies--they should be nearly equal value, but are they different when it misbehaves? Are they equal when it's working okay? How do they respond when the circuit board gets localized heating and cooling?
Use cold spray to "freeze" different parts to see if it "heals" and gets to working correctly. Then use localized heating to cause it to fail or degrade.
What i'm trying to get at is this:
Firstly, the power supplies needs to be rung out to determine whether they are an issue or the cause. An oscilloscope may be necessary because this is a switching power supply with ac signals; be careful with the ground leads to not short or cross the signal to the wrong return, i.e. VL and 12V return are not the same, don't short them thru the scope, don't know what might happen.
After the ps is given a clean bill of health, then ring out the signal from beginning to end to find where to problem starts.
