It looks like nothing has been touched in OSC Mix (V1, V2) or at V3. It is curious that someone was tracing with those markings. I don't see any reason the B+ caps on the MPX would cause this issue.
In my experience, though, those 2 330 ohm 'glass' resistors can fail under heat. I had a 400 that had a burning small, traced it to those 2 resistors, they tested fine but as soon as they got hot, they were failing. This would not cause the popping, though.
I would say your options at this point would be to measure any resistors, or the small ceramics in the OSC mix and at V3, and replace them. Clean up any old flux.
You could try to adjust the bottom and topside of Z1, 2, and 3 but it will affect your FM alignment. Finally, you could remove and inspect Z1, 2, and 3, but again it will affect alignment somewhat.
This might be an obvious question, but have you confirmed that the popping is on all stations and only on the Fisher? I ask because I had an issue where my local NPR station would also intermittently pop. I also thought it was the receiver, but then I heard it on another radio. If you had an FM signal generator, you could confirm whether it's the amp or the FM signal itself.
