Fred thanks for the encouraging words however I should stop talking about in Tech Talk before I get into strife.

A common cause of instability in old radios was the practice of depending on filter electrolytics to bypass RF superimposed on the B+ lines.

Modern electrolytics seem to have a higher internal inductance than the old-school types. Fitting a low-inductance polyester or ceramic capacitor across the electrolytics sometimes cleans up spurious whistles.

Some sets with high gain IFs also needed neutralisation, which was achieved with a "gimmick" capacitor made by twisting two insulated wires together. If that gets pushed out of its original position it can result in instability.

Lead dress is also quite important. I had one circuit using a 6BM8 for the audio amplifier, which used to go into VHF oscillation because I put the screen dropping resistor and its bypass on a tagstrip about 5 inches from the 6BM8 socket, and ran a wire across to the screen pin of the 6BM8. The radio used to mysteriously lose sensitivity every now and then. It was only when I noticed it was interfering with TV reception did I realise what was going on.