Intermittent channel in TT setup - ideas?

Sorry for the long windedness - usually I can figure issues out on my own, but I believe this one is truly "weird".

For the past year, I've experienced the strangest channel cutting-out issue in my vinyl setup. It's now been observed with different phono stages, tubes, and cartridges. Indeed, last time it happened I immediately swapped cartridges and it remained constant - so that's not it. I'm *fairly* sure the dead channel follows the phono cable L/R swap, though I need to verify this next time - that would seem to eliminate downstream gear (i.e. phono stage, preamp, amp).

Here's the symptoms: while listening to music (and it always happens WHILE music is playing), 1 channel will suddenly "cut out" - actually there's still sound coming out that speaker, though perhaps 40-70dB down from the signal level. This must be reflecting the crosstalk limitations of the phono stage, since it kind of sounds like a "ghost" image of the good channel. The problem lasts a few to several minutes while I fiddle with reseating connections, flapping my arms, etc. Signal ALWAYS comes back, though not always on my preferred timetable (i.e. immediately). Once "fixed", I've never had it reoccur in the same listening session. It occurs perhaps once every 2 to 5 listening sessions (~2 hours a piece). Always occurs nearer to the start of a session.

I've suspected the DIN connection from my phono cable, since the plug doesn't fit well into my tonearm's barrel (ugh, this tonearm even has odd square-ish pins). I think that is a more likely culprit than the headshell connection.

Here's the curveball - last night it occurred, and the phono DIN re-seating didn't do the trick. I've always MUTE'd my linestage during the process, but not this time. When I spin up my SOTA turntable (w/ old vacuum pump supply) with mute OFF, it will induce a moderately loud POP (plus a few softer clicks) in my system. So I've always kept mute ON until after the needle's dropped. Since the system wasn't muted here, I heard it - and after that loud POP, I noticed that the "dead" channel suddenly came up in its noise floor to a normal level (typical tube white noise from a high gain system) - as if it came to life. And of course, soon as I dropped the needle both channels were back to full glory. I'd never noticed the noise floor thing before. In retrospect, I think there may have been times with the channel seemed to cut out in response to a "pop" or "thump" sound from the record.

So before this event, I was pretty sure the DIN connection was the culprit. But I'm not sure whether this new information supports that argument. How would an intermitted phono cable connection affect the noise floor - shouldn't that all be fully determined by the chain AFTER the phono input? How would a "pop"-type signal affect a poor connection? Any ideas on what could explain this behavior, or ideas on the issue in general?

Fortunately it's not been a significant factor in my enjoyment of music. Still, I've love to get it figured out!