Another Oil Pressure Guage Question

1992 300E - 150,000 miles

I have owned this one since 20,000 miles and change the oil every 3,000 miles without fail. Lately the guage will often drop to zero - but no loss of power and then later will jump back to almost three while driving and lower while at a stop light. I am really concerned with it dropping to zero. Does this reflect:

a. faulty oil pump
b. faulty guage
c. other suggestions.

By the way my inide is not real concerned with the car having the original timing chain. He suggests the tell tale sign to listen for is a rattling or slapping sound at start up in the morning - whichI do not have. He doesn't seem to push unecessary repairs.

Any assistance is appreciated. Thanks.