Sorry I never replied back in 2012 (LOL)!. We did eventually figure out what the problem was. As I stated, we had a lot of algae. We had to backwash a few more times and add more DE before the pressure stabilized. Since that time, we haven't had another problem. We just know that when we open the pool, we have to backwash at least three to four times due to the algae built up.