No oil pressure/Low oil pressure

I had my car's oil changed on Sunday and I was told that there was no oil pressure so I had to sign a waiver. I took it to a local garage and had it checked out but they have told me that it's not the oil pump and replaced the oil gage. Again, this did not solve the problem so I am wondering what my next option would be as to what the problem is. When the car is sitting the pressure is low but when your driving it's up where it should be and then when you go to stop it goes back down to low. Please help !! What are my options for solving this problem?