2007 Lucerne CXL. and 1995 Lesabre currently, past 1973 Riviera, 1968 Riviera
The gauges are getting the signal from the ECM, the sensor on the transmission may be something to look at if it or the harness is having a problem. It can be another item but not having access or additional information this is a best guess and as mentioned a possible ground issue. Also assuming that, if equipped, the cruise control may also be having a problem.