oil pressure gauge
 

Normally while driving the 400E the oil pressure gauge stayes at the top bar (3) and comes down to bottom bar (1) only at idle. Last week the needle began to swing wildly while driving with steady pressure on accelerator, from 3 to 1 to 2. This week while going 40 MPH the needle drops all the way to the bottom of the gauge and only when I let up on accelerator does it come up to first bar. The mechanic says it is electrical as the needle always starts out at top bar (3) when I first drive car in AM and it is cold.
What is going on and is this a hazard?
Thanks!!:(