Have you tested all three cartridges with the same cartridge-holder, or do you have three?
Test with a multi-meter the suspected wire (white=L/red=R/blue=GL or green=GR) of the cartridge holder. Make sure the small connectors fit snug on the contacts comming out of the cartridges. If not, the best way to make them fit snug again is to put them each in a very, very small vise to narrow their gaps. I pair of pliers works too but you have to be very carefull. Next spray some contact cleaner on a piece of paper and rub the ends of the tiny contact-pins of the cartridge holder V E R Y C A R E F U L L Y clean. again be careful! Here everything can easely fall apart and it is a
job to put things together again.
All Wires okay? then proceed with this: clean with a cotton swab, dense with contactcleaner the silverplated contacts in the back of the tonearm-head. Do not forget the shield contact (a pointy piece of metal wich eliminates eventual Hum due to unsufficient shielding of metal cased cartridges like Stanton 681 EEE)
Next proceed with the wiring and contacts inside- and comming out of the player.
Detach the signal-cable from the recordplayer and test all with a multi-meter.
Wire okay? Then proceed:
Clean the surface of the soldered contacts of the tonearm wiring. Why? As you might know: electrical current of low voltage (mV! it is here) is almost only surface transported.
That is all there is to do. You can leave the short-contacts wich act as a noise canceller during record interchanging, and or automatic shutdown, out of the question.
If this all did not do the job, you may suspect very reasonably the tone-arm wiring itself, but from my 30 year or whatsoever experience with DUAL I can say it is rare that the tonearm-wiring on its own is the cause. or someone before you (you never now) has been fumbling around with you DUAL.