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If there are any original tubular capacitors in there they should be replaced--blocking and supply-bypass alike. If a leaky one is loading down the B+ to the FM front end it can sometimes manifest itself that way. A global replacement of those caps may solve your problem.
If the problem persists then look for the simple and obvious things first.
A schematic is a good idea if it is evident that someone might have replaced any of the disc capacitors associated with the local oscillator. Many of those have temperature coefficients. Oscillators are critical in nature. If the wrong part is in there you can get severe drift. Get the Sams and look at the component list and make sure everything is exactly as it should be.
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