Oil Pressure guage wacky

HI I am new here and I need a answer to a problem. I bought a 1998 Dodge 1500 Van 318 had the oil changed last week and I noticed that the oil pressure gauge drops down to no pressure after the van has been running but it goes back up to normal when I am driving and goes back down when I stop the check gauge light then comes on. I have oil in the engine and I just had the oil pressure switch replaced I am at my wits end can someone give me a solution. Its starts doing this after the engine warms up. Julie