great flow then nothing...

Help!!! So I have an above ground pool with a new 16 inch Haward sand filter and 1HP motor. When I initially start the pump (after rinsing the filter) the pressure was a bit high at 20psi but the flow was awesome. The problem was the flow slowly diminished to almost nothing in 30min - 45min consistantly. So I was thinking air in the intake. I looked and there was a pinhole inconsistent stream of air that no matter what I do I cannot make it stop (more teflon tape and even changed fittings).

When I back flow and rinse the filter the cycle starts again. I cannot believe the filter is clogging so quickly and if so I am not seeing the corresponding increase in differential pressure that you would expect in a clogged filter. It is almost like I am on a centrifugal pump float at 20psi. I did not the pressure guage is not stuck you can watch go from 0 to 20 psi and when the filter is back flowing you can watch it bounce around 5 psi.

Thanks in advance,
Peter