Decrease In Pool Return And Increase In Pressure Every 2 Hours

[pinfish]

New to the forum. Just bought the house with this pool last fall. I have a 20,000 gal pool. Pentair pump (new impeller about 1 month ago) , 1.5 HP motor SF 1.5 (the motor is brand new and replaced a 1.0HP SF 1.6), Tagelus TA 60 filter- sand is 6 years old. Polaris with 3/4 HP booster pump. We have had some pavers installed to fill in some palm islands. I noted some sand in the pool prior to the paver install but assumed that it was blowing from the palm islands into the pool. After the pavers were installed there was a lot of sand in the pool. I know that they got a lot of sand / paver dust in the pool during the install. I have been been working a sweeping the sand toward the main drain and running the polaris. I have backflushed the filter. When I first turn on the pump the pressure reading is about 22-25 and there is very good flow through the pool returns. After about 2 hours there is almost no water coming out of the pool returns and the pressure gauge on the filter valve reads about 35. If I turn the pump off for 30 sec and then turn it back on the pressure again reads 25 with good flow out of the pool returns. What could be causing this? It seems that as hard as I work, the sand keeps recollecting on the steps (one of the pool returns is close to the steps). I have held a white rag over the pool return and I don't see any sand coming out. The only other modification that has recently been made is in the backflush line. I had the 20 ft blue tubing running out onto the lawn. I recently had a 1.5 inch pipe run under the ground to dump into the canal behind the house. This line is about 40-50 ft and ends about 10 ft lower than the bottom of the pool. I know that this is a lot of info - I'm new to pools and dn't know what is important and what is not. Thanks!

[pinfish]

OK - I just read a post about the sand in the filter. Even with a lot of paver dust/ sand should I have to backwash in 2 hours? Can any other info about the situation be obtained from the post. I am new to pools - I don't think that I have a vacuum line.