@CarlD nailed the pump/filter sizing issue. 30' of head is approximately 13psi pressure. What's your normal system pressure?

Also, I'd like to chime in on the DE issue. Your original post stated that DE was coming back through your return lines into the pool. I took a look at the filter manual and there is a built in check valve in the filter. The troubleshooting section clearly states that DE returning to the pool after pump shutdown could be a result of a bad check valve.

Have you disassembled the discharge elbow and confirmed the check valve is operable?