I think some genius on AudioKarma suggested it might be a fault in the muting relay, but he is delusional

Well I'll take that with a grain of salt then.  

Just wondering why a quick power off fixes it for a long time.   Probably not heat, maybe a capacitor.   I think the muting relay is to turn off the input when parked, that works fine.

The muting relay does not break the signal paths.
It merely grounds them.

There are no electronics in the signal path at all.
Any sound related problems will have to be caused by the cartridge, the internal and/or external signal cables
or the amplifier and speakers it connects to.

Martin

Thanks Dillen!

Well I only tried two different carts in it.  It is possible they are both weak as they are old.

I swapped the cables and finally the right channel went lower in volume.  Which makes it something from the Cart to the phono preamp connection.

I wonder why cutting off the power to the TT for 10 seconds fixes it for like 5 hours of play?    If its the cartridge do a voice coil develop a charge which is then sort of reset if you power it down for a few seconds?   Just trying to understand why it develops over a period of play time vs just being that way when I play the first record.    Usually takes hours and the fix takes a few seconds.

RAJOD wrote:

I swapped the cables and finally the right channel went lower in volume.  Which makes it something from the Cart to the phono preamp connection.

Or the preamp or the amp

No not the preamp, I have tested with 2 different preamps same result.  And other components hooked to amplifier CDs, other TT run fine without channel drop.

Bad connection, solder or pressure. Cracked insulation... Making condensator to somewhere on the line... ?

I have a third cartridge I can try, that will rule out it being the cart.  (I hope its the cart)