[morzh]

If there is a squeal, the osc could be OK.
At any rate, I would not touch coils before I exhausted the freezer/poking.

Here's the thought train:

1) it quits after long time working. Implying, temperature could be at play.
2) What could temp rise do?:
a) cold solder spot disconnect.
- test: take a chopstick or a thin plastic rod, start poking (not too gently, a poke has to have physical effect) at every soldered cluster, every screwed lug connection, every grid cap, wiggle every tube, poke every knob....anything that is not the chassis itself, but soldered/bolted/etc, has to be wiggled/poked.
- test: same places, freeze spray (not too much, just cool it a bit, like spraying from short distance in 1/4 sec pulses).
b) internal tube electrode short.
- test: tap the tubes. Take the grid caps by the wire, try to gently apply rotating torque to it. Very gently, you want tension but not actual rotation. DO NOT use freeze spray, you might crack the tube.

3) You said Candohm is good. Candohms could go temporarily bad. Try to wiggle Candohm's lugs, use little Freezespray on it, measure the voltages during the "quit" period.
4) Simply wiggle the band switch knob lightly. Or try switching bands back/forth. Just for the heck of it, wiggle tone ctl switch.
5) last. Wiggle the speaker wires, the cable that goes to it. Tap the transformer, poke with the rod the wires that go into the field coil.