Tonearm, or RCA? higher up... occasionally no sound in left channel.

This is driving me bats. As regulars will know I've had to downgrade my set up and this 2nd hand sondek was one of my purchases. First I thought it was the phono stage, then I swapped the cables round L to R, R to L, and the problem shifted from left to right, meaning its downstream of the phono stage. I also gave the RCA sockets a little pinch to make sure they fitted okay. Tonight I've swapped cartridges over, and it still does the same thing.
I thought I had it nailed at one time when I put a slight loop on the white cartridge wire at either end of the tonearm. It worked for a night, now its gone wrong again. The shop I bought the phono stage from said its very rare for cartridge wires to snap in the armtube but not unheard of. Could even be a crap tag. So my next victim, is it the armtube wiring or the cabling inside the turntable. Do I buy an ohmeter whatever one of those is. The arm is a Hadcock 242, and the TT is a linn sondek, an odd combo, granted - but when it does work... The cartridge tags are one run from the cartridge through the tube and to a small termination block that enables you to switch armtubes. I'm not sure I'm clever enough to put a new tag on my tube, and don't even know if it needs one. Thanks! 1.10am, fed up. PS, how do I get to the tonearm cable inside the Linn, never had this nonsense with high mass TTs!