[MrFixr55]

the issue of probing anywhere on B+ causing the volume to restore to normal is strange indeed, as a 10 MOhm load on a power supply that has to deliver about 40mA at 250 VDC is not going to affect much in the line of disturbance.

Try the "Divide and Conquer" method of injecting Audio from an MP3, CD, etc. in at the volume pot. Not at the wiper but at the input to the pot. If the radio already has a "phono' jack, use that. Otherwise, consider putting one in. If this issue happens on injected audio, hen the issue is with the Audio and Power Supply. These circuits are simpler to troubleshoot than the RF / Detector / AVC section.

One possible issue is the speaker. It is not uncommon for the flexible leads from the voice coil to the speaker terminals. Although it is more likely to cause static especially on low notes or loud low voice talk, cut in / cut out is not uncommon. The "spike" from touching B+ may cause this. If this is the issue, the fix is simple. the wire is like a braid and flexes most at the terminals. add solder, let it go a little further down the braid. This may solve the issue.