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My guess is that the problem is in the cathode and grid area of the 3.58 MHz oscillator. If possible measure the component values, especially the cathode inductor L42 , its capacitor C213, the 4 pf feedback cap C209, the crystal and its resistor. Or maybe some conductive goo on the tube socket pins? My 21CT55 oscillator cathode tuned coil had been mis-adjusted and caused the oscillator to stop often.
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