I'm not sure whether anyone can help you or not -- obviously, there are problems with your circulation system, but it's going to be very hard to describe what you've got adequately with words.

You might consider taking a careful, systematic, set of photos of entire piping circuit, and sending those to poolforum AT gmail DOT com. I'll upload them to the PoolForum Picasa album, and then link them in.

I will note that 18 psi is a very high starting pressure on a DE filter. I'm curious why that might be.

Ben