I used to have this exact problem with my (sold) US-300D.

The problem was indeed grounding, amongst other things. Make sure the instrument cluster has good contact with the chassis, as well as the battery being grounded to the chassis. If it isn't a grounding issue, pull the cluster and take it apart. Check the PCB board for any breaks in the electrics. In my case, right by the 14-pin(?) plug, was a seperation in the solder-tracking. Get a nice soldering iron heated up, and fix any breaks in those tracks.. and test with a multimeter.

The most common thing I've seen is poor grounding, and electrical breaks / shorts. Check those, and get back to us with the results.