1992 Ford Bronco Low oil pressure

I have had my truck for three years now. I have changed my oil when it's supposed to be done an have even went with a thicker oil. My problem is below 1100 rpm my oil gauge does not give me pressure. Above that it's fine or normal. The guy I bought it off of said he replaced the motor at 150k. He also gave me a short list of the parts he replaced already. All of which I have had to go back an redo an then some. Is it possible for a bad gasket or no gasket at either side of the pump cause this problem? Or just a bad pump? An one more thing I have replaced the oil pressure switch on the block. I find it hard to believe that he did replace the motor considering it really doesn't have that much power to start with.