i seem to remember years ago my 91 300e had something like that happen i pulled the gauge cluster and found a bad connection on the board by the dimmer knob i think(it was a long time ago) but it was something i was able to resolder and it has worked fine since. i know this is vague but i would start looking for voltage at the dimmer knob in the gauge cluster the gauge cluster in the 300e is held in place by 4 spring clips that can be released by inserting straight flat tools like a small putty knife or old kitchen butter knives and then disconnecting the speedo cable when you pull the cluster forward