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?

Hi Neil - Without seeing what's going on first-hand, I can only offer up the following:

First of all the channel drop off - interesting you bring this up as I experienced this with one of my previous project tables.  Most notably the "fix" you mentioned was exactly what I did as well - what turned out to solve this for me was switching out the cables from the TT to the phono stage.  All I can guess is that the email connectors on the project were too large in diameter and weren't always making a connection.  I've since moved up the Pro-Ject food chain and haven't seen the problem since.

The rumble could take more troubleshooting and my suggestions are based on an assumption (not clear from your post) that it's happening even w/ no music playing correct?

If so, I'm going to guess that it's low frequency resonance between the tonearm and your subwoofer at the arm's resonant frequency below 20Hz.  An easy test would be to turn off the LF filter on your phono stage, and with the volume up (nothing playing) go thump on your table with the sub on versus off and see if that makes a difference.

If you find that it is your sub, then I would either opt A) not to use it when playing records B) adjust it so it doesn't interfere with the tonearm C) try better isolation for the turntable itself which is always a good idea anyway

Good luck

You could purchase a KAB Rumble Filter from KAB.  It solved the problem with my Scout turntable and there is no loss of air, clarity or otherwise.
There's no way interference of any kind would kill one channel. Process of elimination is your best route. Do you have a local audio buddy who can let you try their cables, phono, phono stage etc. so you can narrow it down by swapping one at a time? 

You shouldn't need to buy any filter to eliminate rumble, if isolation doesn't help, and the table is that bad isn't there a bigger lesson to learn about the table? 
Even buying a vintage $20 craigslist table will be cheaper way to evaluate your issue. You could probably resell at same cost if you wanted to. Cheers,
Have a tech take a look at the phono inputs. Perhaps you have an intermittent short inside the chassis due to a cold or broken solder joint. Likewise, I would check the connection between the ICs and the tonearm.  What connector is used at that junction? Is it a DIN type? Check inside the tonearm, in that case.  Also, check the cables themselves, at both ends.  This is easy. Just unscrew the cosmetic covering over the solder joints and inspect.  Anyway, this sounds like a simple problem but one that can be hard to pin down.

Thanks for the thoughtful responses!
I switched out the Phono to Pre RCA's last night and the problem went away. So in all likely hood it's a fault in the cable; although i did check for continuity a few weeks ago with a multi-meter and seemed to check out fine. A lewn pointed out it may be an intermittent fault in this cable. Will keep you posted.

sbank: Yes i have my doubts about the Pro-ject table. In all honesty i bought it as a stop gap with full intentions of moving on within the next year. Possible candidates are VPI, ClearAudio or Kuzma... any thoughts?

What's your budget and preferences in a table? Do you have any direction in mind(e.g. I always like Brand X cartridges, or I definitely want removable headshell so I can easily swap in a mono cartridge)? Cheers,
Budget is  < $2k including arm but excluding cartridge.
Removable headshell is not something i had really considered. The table should be set and forget.
I had a tricked out LP12 in a past life and the constant setting and resetting drained all the joy out of the experience.
Aesthetically (which is important as the table is front and center in the living room) i prefer clean and simple
Sounds like you need to look no further than Rega for your set and forget table.  Depending on cartridge choice you may need to get some arm board spacers.  I have owned the P5, P7, & P9 along with cartridges Rega Exact, Dynavector DV-20XH, and Lyra Delos.  Very fine tables.