If the problem is temperature sensitive, that tells me almost certainly we're looking at a failed/failing capacitor. Where that capacitor is located could be the mystery.
If the other member changes his mind, please let me know.
I'm certain you know this already, but if you remove enough sheet metal to allow you to see the component parts, please look for capacitors which are swelled, bowed at the top and/or bottom, leaking anywhere (you will usually see a whitish stain on the phenolic board) but most commonly at the bottom...anything like that. And these will nearly always be electrolytic capacitors (look like a tiny beer can in ways)...although today some of the newer variety of capacitors (non polarized) will be made to mimic this same look. The trick comes in the integrated circuit chips. Those will contain transistors, diodes, resistors...and capacitors. When the capacitor part of those fail, the chip is toast and those can be devilish difficult to diagnose and replace. Often you simply can't find the right IC. Electronic supply houses sell chiller spray. You can use that while the radio is powered up to test capacitors. Spray each one a bit and if the radio starts working correctly, you have found the culprit. Recently I purchased some pre-made circuit boards from a Chinese company for a test project with which I'm involved. What I learned was surprising...one board had a defective variable resistor but the rest of the circuit was ok. I figured I could just replace it and move forward, but the Chinese had built it on one thin and flimsy printed circuit board and then had laminated a thicker and more durable board to the bottom of it, making a phenolic sandwich. So, you could not un-solder and change any of the pass-through mounted parts. For what it is worth my uncle made capacitors for use in radios during the 1940s. A lot of people did that as a cottage industry (lunch money, basically). The war effort had depleted the supply and makers of radios were contracting with people to make them at home for pennies each.
