Is the pool cleaner a pressure side or suction side cleaner? If it's a suction side then we look at the cartridge itself. The simple fact that when you clean the cartridge the pressure drops seems to indicate you're trapping something. Just to check, try running the system without the cartridge and get a reference point pressure reading. My guess is it will be way below the 18 psi you mention after a cleaning. Just looking at a filter with the eye isn't going to show if you have really fine stuff packed in and this isn't coming out with a normal cleaning. You'll eventually have t get a new cartridge so you may just do tht now. If that doesn't solve the problem just put it on the shelf for future use. Are you battling an algae problem or some kind of a dirt problem? I've never dealt with cartridges so maybe someone else has other ideas.
Al