1992 Ford F-150 oil pressure problem

I just replaced the engine in my truck and I have recently notice my oil pressure gauge is acting funny. The engine was sitting for a while prior to installation.
I was leaving work and when I started going I looked down and noticed I had no oil pressure. As I went along the gauge made it's way up. Finally it got to where it normally would be but when I stop it would drop again. I swapped the pressure sending unit from the old motor and goyt pretty much the same results on the gauge would also read higher than it was with the other unit. I checked the oil and it is right where it needs to be. Any suggestions?