Hi I have an above ground pool,and have a pentair st35 filter system, my problem is I’m losing pressure going into the pool, and the pressure gauge goes up to 20 psi , I replace filter grids and clean pump and outlet , hoses and still the same , I open and wash out the filter put it back together run the pump fill DE in and it runs great for about 30-55 minutes and the pool pressure fades out and the tank gauge goes up to 20 c I tried all I could nothing works, please help, what could be the problem? Thank you for your help.