In 1992 (don't know what month; will look it up tonight), in an edition of the "Star", Frank King relates several instances of oil pressure gauge problems related to voltage issues. Replacement of the voltage regulator cured the problems in those anecdotal accounts and mention was made of checking/replacement of the overvoltage protection relay. FWIW.
Morris, check connection to the sensor. Open contact results in a constant 3 bar reading.
EDIT: I was wrong (no big surprise there
) March/April 1993 issue and it dealt with temp. gauge. The basics of the article still apply