I have a DT filter thats a bit old but still works fine to my knowledge. I've been having a pump issue as well but I don't believe its causing my filter issue.
If I run the pump my filter pressure goes to 55-60psi. Then if I backwash, while I am backwashing it goes out at 15-20psi. THen once I backwash and turn the filter back on normally it works at about 35 psi with good flow for about 30 minutes and then it shoots back up with bad flow to the 55-60psi range.
Is it possible that there is an obstruction in the outlet? I can't figure out how this would happen....