I spent most of the last two days experimenting. My oscillation when using that bypass capacitor was due to attaching the ground return from the pot to the audio input of the AGC board. With the bypass capacitor in place (10 nF on my test, 470 pF on Kees' board) the resistance of that return wire afforded enough of a voltage drop to create a feedback path.
If I were to encounter RFI or audio feedback I would heartily recommend a 10 nF capacitor across the audio input of the AGC. If you encounter a squeal, then open the ground return from the volume control pot right near the board and do all that is possible to make a low resistance ground for the 5 volt power return (as well as the RF).
My sincere thanks for all of the work that you folks invested and please accept my apologies for failing to anticipate the problem. 73, Don
Don, ND6T