jrseattle,
I wonder if C776 is leaky. That will cause problems if it is leaky. This is a part that can load down the circuit. Those I always replace with a film type. You said a problem could be to the left of R777 which is where this condenser is.
The 741 IC, U740, could be bad when it warms up. I had that happen recently. It caused readout problems when it warmed up a couple of deg. C. 741s can fail in odd ways at times. Changing it with another one or any that will be a sub might be the problem. There are other numbers that are considered better. Looking at the specs of the ICs and the circuit will tell you if you want another number. I have tried FET input types in place of a 741 and the circuits worked fine.
Ozan is right about the TIP142 as a replacement.
Mark
