I have been trying to diagnose an erratic oil pressure gauge this year. Even when the oil was clear up to the FULL line on the dipstick, just the slightest hint of a change of direction of the truck resulted in the OP going to zero. The engine wasn't making any horrible noises. So I thought maybe there was something wrong with the gauge.
Today I did an oil change. 5 quarts and a new filter. I started the engine and let the oil circulate then shut it off and looked at the fancy chrome dipstick my engine builder used. A full 5 quarts on it registered at about 2 inches higher than the full mark. I checked the amount of oil I drained out and it was about 2.5-3 quarts. Ya. With a full pan of oil, the gauge is rock solid and the engine seems to be a little quieter.
Check your dipstick!