One big thought to start is how simple the canister filter really is and that makes it nearly impossible for our first thought to really be true! We tend to jump to something being wrong with the motor, etc but those are so reliable that I find they rarely fail compared to other mechanical problems, even when those problems are hard to spot. Just have to hang in there and know that there is "something" blocking flow!
One really easy to happen but terribly hard to find is the intermittent type blockage as you describe. So look for something moving? Things I have found on all filters is shifting media and one of the hard ones is a part of the hard stuff like a barrel that gets away and falls into the can, where it can sometimes move into the tubing, hang and block flow but then move out when we are looking but we still don't see it! A second one might be to check that something like a pad is not allowed to float up against the space where the flow goes up and out or other space where flow might be blocked either by floating or shifting. Easy to turn the grates the wrong way up if it is like other Eheim, perhaps a check of those, even when we are certain we are right?
Killer to find but easy to fix---after you pull your hair out!
Things like shifting the tubing so that it kinks but straightened when we are looking?
Sorry for the "no good answers" answer but it's just a bummer!
Last edited by PlantedRich; 05-17-2019 at 04:20 PM.