It sounds to me like you are getting some really nasty RFI into the sensor wires. Somewhere arround the engine there is a vibrating metal to metal contact that is putting out a lot of RFI (radio freq. interference). Glow plug clip could be a candidate. I assume you have a plactic/nylon connector on the throttle arm. A bad engine bearing can do this also. The other thing you can try is to wrap the sensor wire arround a EMI filter (metal torrid often suppiled on the power cord of audio and video components. I think you also told me you were running glow engines - that eliminates spark noise getting into the sensor wires.

To verify this is the issue you can route the sensor wires outside the cowls and run the engines and see what it does. If problem is gone then you have verified that it is RFI being generated somewhere arround the engine.