1994 Dodge- 1500 Ram V8

My pickup has 134,120 miles on it. I had the oil changed and my oil pressure started dropping to 0. I can be driving and as long as I turn the OD off, the pressure will stay normal, til I stop at a red light or something. Then it drops to 0 again. If I leave the OD on, the oil pressure will jump up and down for awhile, then drop to 0 and stay there. I took it to the dealer, they said that I had internal engine problems. They never hooked my truck up to anything, nor did they even look at my truck. One said it was because I am a woman. I don't buy what they are saying, I think the oil pump is going out. The sensor is good, the oil level is good. Do you agree or is there something else I should be concerned with? I had no other problems before this to indicate internal problems.