[AI2V]

If tapping the tuning cap does something as you note, try tuning to a station at the very highest point on the dial (minimum capacitance in the tuning cap) and see if it happens there. If the problem goes away, you probably have a shorting plate in the tuning cap.

If you problem happens with a minor rotation of the tuning cap, you may have lousy connections in the tuning cap rotor wipers.

A third thing to consider: a temperature sensitive resistor will always look just fine when measured with an ohmmeter; it's only when it's hot and current flowing that the problem will be seen. Let the radio heat up fully and when the problem begins, start spraying each resistor, starting with the largest wattage, with some freeze-spray. You can also speed up time by heating the underchassis with a heat gun- but not too much.