Thorens TD110 - flaky tone arm socket??

My trusty old TD110 has developed an infuriating problem lately. It seems that the 4 connectors in the socket that holds the tone arm have stopped making a good connection consistently.

I removed and re-inserted it and it worked OK for a short time, but now it is silent again. I tried my usual tricks like cleaning the ends with a q-tip and a pencil eraser, but it isn't helping. If I leave the amp on while I remove the arm it does make the normal noises you would expect, so I am reasonably sure that the wiring is good from the socket to the amp.

I am going to try my continuity tester tonight, but can anyone suggest some other tests to do to get it working again?