[RodB]

Your IF transformers are tuned to the resonant frequency they were designed for. When you observe the bell curve they display, the transformers pass signals at maximum voltage at this frequency. Sometimes it's too high and creates an unwanted oscillation. The fact that it occurs with distant stations seems to be the clue because the AVC has biased the rf gain of the first three tubes very high. Detuning the IF slightly will flatten the bell curve, reducing the gain slightly at resonance. When you do this it may be covering the real reason for the oscillation, such as a bad ground connection or mistaken capacitor value in the AVC (bakelite block part 7).