Sand Filter Pressure

[dj000524]

Just bought a house with inground 20000 gal. pool. Have been here for about a month and noticed a new issue that I am trying to figure out what is going on. After I backwash and rinse the filter I notice that the gauge reads around 16-20 on it. I have been told that is about what it should be reading. My question is the next day the gauge might be reading around 25-30. That seems high to me. After reading several posts I have seen that backwashing shouldnt been needed but maybe once every two weeks. But after 2-3 days the gauge is reading around 35 and bouncing almost to 40 and there is no cirruclation in the pool. After a backwash I can watch the water move around the pool. Once it gets high on the gauge theres not much water movement. I am beginning to believe that I have a partial clog somewhere underground. Anybody have an idea what it might be or have a solution. Thanks for taking time to read and replying.
DJ