Intermittent Left Channel Outage-- Older McIntosh System--Love my system appreciate Help!

I would greatly appreciate any suggestions from any of the bright people out there regarding a sporadic, but consistent, left channel cut out on my older (1978/79) McIntosh system.

[The system consists of a Mc 2125 Amp, a C28 pre amp, a MQ 101 Environmental Equalizer (used by McIntosh in conjunction with Mac XR5 speakers), and McIntosh Speaker control relay box.  The order of things is: streamer to preamp, preamp to both speaker control relay box and MQ101--  MQ 101 to Amp and then amp wired to speaker control relay box, which is then wired to separate main (Mac XR5) and remote (Definitive BP20) speakers.  Also in the system, but not affecting the cut out issue are a Bluesound NodeX streamer, a REL S510 subwoofer, and McIntosh XR5 loudspeakers.  BTW-- I know the speakers are not the issue since I run a second set of remote speakers (the Definitives) which cut out as well.]

The issue, which follows a consistent pattern is everything works well when first turned on.  After about 10-15 minutes of listening (streaming Tidal from the NodeX) the left channel cuts out.  However, if I quickly push in either the main or remote speaker switch on the preamp, I hear a static "pop," and the left channel comes on again.  After another 5-10 minutes the same thing happens (left speaker cuts out) and another static pop from pushing the main or remote speaker switch restores the left channel.  However, after another 5 minutes the left channel goes out and the static pops do not restore it.  However, if I leave the system on for a couple of hours and come back and create another static pop by pushing the main or remote speaker switch the left channel returns and then will run flawlessly as long as I choose to listen.   I do not think the issue is the preamp, since I run a line out from the preamp to a Soundcast (outdoor speaker) wireless network box, hence bypassing the amplifier, and the left channel works perfectly.  I've taken the system to a McIntosh repair center and they couldn't duplicate the issue.  On my last call they thought the issue was the pre amp, which I highly doubt since I can send a signal wirelessly from the pre amp to my soundcast outdoor speaker and the left channel does not cut out.

Sorry to be so wordy.  Not sure how to communicate this without stating all the "facts."  BYW-- after 45 years the system is finally in the right acoustical setting  and creates for me a better than you are there listening experience.  So don't want to ever give it up.  But that's another post!  Thank you in advance