When you said re-capping I assumed you meant just the power supply caps, (as that is what most people do first off) since
all caps were not mentioned, I know, I know, getting a little technical, but I cannot read between the lines and have to make an assumption here and there till clarification. Now you state after it was up and running then it changed, because of that statement I was assuming you didn't change all the dubious parts. All electrolytics and
all paper caps defective or not.
Did you check for hairline cracks in resistors? corrosion in tube sockets? Dirty tube pins?
There are a couple of items that should be addressed, Not all replaced components are considered as working as intended. Many new parts fail by breaking down after replacement, so, even though you say it worked, is no reason to assume that's still not a problem area. Always consider new replacements, will and do fail often. So back to the drawing board. Testing of tubes is a weak link also, as they may check out fine on a tube tester, but fail under actual voltages and loads, A tube tester cannot provide a load to the plate or operating grid voltages the same way, as when in the actual circuit. They however can simulate some parameters such as a 60 cycle or 400 cycle signal to the grid, not to say there aren't tube testers that can accomplish that.
Cold solder joints fractured solder joints and connections are always a possibility.
One big culprit is heat, and after you said for two ours it played OK then went into distortion, can happen as everything heats up, even solder connections that look good, Transformers heat-up and if to close to upper design limits can breakdown and cause internal windings to short, capacitors and resistors can change values even when new if they are cheap. Check the input voltage especially when working with older radios, Higher line voltages can effect radios, there are just so many variables its hard to pinpoint in a forum easily. Be open to speculation as I cannot know exactly what you did and did not accomplished using generalities, even though you think you were clear in your explanation.
I am used to using details in my previous work, and had to follow precise written instructions, then we had to explain in written detail even which parts were tested and to put down the measured values,(voltages, currents, signal strengths, dBMs, calibration tolerances, operational parameters and test equipment used and required and their calibration dates and expiration's, all in triplet and even more copies to make a paper trail.) before and after changes.
I always made suspect and resolution notes for my own future reference because It helped to see if there was a pattern developing and common errors, no stone was left unturned
. Physically being there, One on one is always easier, However explaining by forum isn't always the most efficient way to go, but since you haven't found the reason for the conundrum you are facing, one would have to start to make assumptions based
on our own personal experiences, and apply the same methods by asking and double checking.
In closing just remember we are here to help and some times its a slow diagnostic process, and other times it can be quick.
Did you do a comparison of voltage checks before the problem and after the problem arose?
I would, by reinstalling the correct tube and see if problem is evident to start with or if it again goes a certain amount of time before developing symptoms. If it is already present when using the old tube then I would retest the tube. Just to make sure that the tube wasn't damaged. More detective work ahead.