Fender Hot Rod IV - works fine, then ramdom cold bias (-106 VDC)

[spunko]

Hello friends.

This is how the amp fails: first power on, works perfect for 3 hours. Power off, let cool  for 2 hours, then power on again and the amp sounds super flat, distorted and low output. This happens like 3 or 4 times in a month of heavy use, says the owner. To solve the problem he powers it off/on again and works fine until the next time.

When I got it I couldn't replicate the problem for like 2 weeks, and when it finally happened I noticed that both power tubes had -106VDC of negative bias on PIN5. Then I tested on the BIAS pot and the same -106VDC. Next I went to test on R76 but I lightly touched C39/C42 in the procces, and the voltage came back to normal -48VDC.
So I suspected a cold solder joint, but I decided to replace R76 anyways. Also reflowed all components on that section. Tested the amp and it worked great.

Now the amp has been working with the same hours of use per day for the last two months, but today the problem came back. Same behavior, used it for 3 hours, let it cool and fails. Power off/on and it works good again.

I don't have the amp with me yet, but I'm thinking of what could be wrong. I was planing on replacing also the diodes and the caps, but I want to be sure of what could be happening.
As I understand, R76 is droping from -106VDC to -48VDC, but replacing it and reflowing components didn't solve the problem.

Attached is the diagram for the III version.

[sluckey]

Bias pot and R77 and/or associated solder joints are prime suspects.
 

[AlNewman]

So the voltages line up with the schematic, only not at the right places.  Does the switching and reverb work when it malfunctions?  Something isn't drawing enough current when it fails.  Maybe a bad solder connection leading into the op amps?  Especially the 470R dropping resistors.

[spunko]

I forgot to mention: C37, C43, R78 and R79 were also replaced.
I resoldered all the components from C37 to R78 and R79.
Switching and reverb work fine.

[sluckey]

Bias pot and R77 and/or associated solder joints are prime suspects.