Oil Pressure

I have a 1999 Dodge Ram 2500 5.9L 4 wheel drive with an automatic transmission, which has 114k miles. My question is, what would cause my truck to drop oil pressure and how would I fix it. What is happening is when my engine heats up and I come up to a stop sign or I slow down my oil pressure gauge drops to zero and when I rev it it will go back up. Also my oil pressure gauge doesnt ever go above 40 plus the needle wont even move when I rev the engine. Also I have no visible oil leaks. And I have done an oil change and everything and still the same problem. Your help would b greatly apreciated ty.