My gauge was doing weird things too and when I bled my system and refilled with coolant it seemed to stop working completely! I thought I had done something seriously wrong or there was something drastic wrong with the bongo
Helen gave me a pointer - she told me to check the voltage stabiliser wire at the back of the dials. I was going to just replace it but I took the dials out and loosened the screws where the wires fix on and tightened then up again. I pulled the male/female connector apart and reconnected. I also tweaked the sensor on the engine. HEY PRESTO - it's working again! It might just be a loose wire, so check the connections first before you go spending huge amounts of money on new parts!!
Hope this helps.!
BeastieBob