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My first thought is that you probably have a component, such as a resistor or a capacitor, that is failing. It's in good enough shape to allow the radio to power up and operate, but as the bad part gets hotter and hotter it begins to drift farther out of spec, as evidenced by the shifting frequency, until it gets so far off that the radio no longer operates.
The first thing to try is to take the chassis out of the cabinet and do a visual inspection. Look for any parts that show evidence of heat, leakage, bulging, etc.. If you find something that makes you go: "Oh-oh, that can't be good!", then that's probably it.
While you're in there, I would also put contact cleaner on the tube pins and sockets. Sometimes a poor connection can cause a tube to go out of its intended operating parameters.
If nothing is noticeable, then you need a schematic and a DMM to start checking voltages throughout the circuit until you find where something isn't right.
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