I have a DE filter for my above ground pool.. A lingering problem exists?

My wife and I put the filter and pump on with the chlorinator... Our water had a lot of algae in it.. After going through the process, we have most of it all out, filter comes out clean on the filter, problem is the filter stops pumping the water back into the pool every 20/30 minutes.. We have to go out, stop the power, prime the pump with the handle, then turn it back on, then it blows the water back out again like it's supposed to. We clean out the fingers every 6 hours to check.. Te pressure gauge reads 20-25 when we shut it down, then under 20 when we start it again... Any ideas?