When the volume drops, and then comes back up, does it then stay up? Or does it keep going through cycles?

It'll take a while, but you can monitor the plate and other voltages of each tube, and see if one of them drops when volume drops. I would start with the oscillator section and then the RF and IF sections. The audio section may be OK, since you hear hum when the volume drops.
Doug

::Now, come on you guys! someone must have some ideas about where to start looking. Anyone? Mark from Kalamazoo
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:::Hey, all. Looking for some ideas about a problem with this radio. Sometimes (but not every time) after it has been on for a couple of minutes or longer, the volume will start to drop until there is just a hum. After a few minutes, the volume slowly comes back up and the radio plays normally. I was hoping that the problem would worsen so that it would be easier to locate. I hate intermittent problems like this. I have replaced various resistors, paper caps, and the electrolytics in previous repairs. Any thoughts about what to look at first? Mark from Kalamazoo
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:Mark:
:Are you married? If so, does your wife have a hair dryer? Do you have a Radio Shack nearby? If so, do they have cooling spray? Assemble the preceeding while study the schematic and we'll get together later.
:Lewis