Cogging/9920 Problem

[Nick]

I have started to get this quite often now.

I can use my truck and if I slow down or stop I can't accelerate again. It just won't move and the 9920 makes a very quiet humming noise. I can't stear either (which might show it isn't cogging because all electrics stop).

I have to unplug the batteries and plug them back in again.

I run a 7XL & 9920 with 20/66 gearing. It's a been cold when I've run it and nothing gets hot.