Noise, rumble and general intereference

Hoping a few of you might be able to shed some light on a persistent problem i seem to have with my set up and suggest a path forward?
1. Gear: Speakers (Gallo strado with REL sub), Pre, Dac and Amp (All Bel Canto eOnes), Phono (Cambridge CP2), Turntable (Project RPM 3 with Blue Point 2 cart)

The pre, amp and speakers sections have performed flawlessly for many years. Up until a few months ago i had a digital only system streaming music from a server or spotify. I recently added the analog front end. I had a great analog rig 10 years ago (linn lp 12 and EAR) but moth balled that system due to life style changes.

The issue I'm having is a) low level rumble which i can reduce by employing a low level freq. switch that the CP2 has - although this does reduce the overall space / air in the sound).
However far more vexing is the problem of intermittent channel drop off, and by that i mean when i play vinyl occasionally one of the channels goes dead.
Initially i thought this was the turntable so i had Pro-ject service it (i had purchased the deck used on agon) but Pro-ject ran tests and found the table and cartridge in perfect condition.
I also know its not the phono as i similarly returned the phono and had the manu. send a new one... still same problem.
What does seem to correct the problem for a few days is if i switch the RCA cables (literally unplug right channel and plug it into the left or vis-a-versa) but then the problem comes back. I think it is some sort of interference.- the phono section sits on top of the DAC (could this be it) The cables from the deck to the phono are shielded and grounded and so are the RCA's from the phono to the pre.

Any insight would be great?

Thanks
Neil