It might be a block check valve (but the "bang" temporary solution wouldn't match that -- maybe that's a coincidence.

My guess -- hair or residue on the suction side. The colapsed hose is a possiblity. An air bubble, maybe linked might be an issue.

Larry: Try to backflush the shower sump. If it has a screen, remove it before you do it; and, don't douche the inside of the boat with your hose. ;^))) You might have a screen in the line that someone put-in is a stupid, hard to reach place?

The positive displacement pumps (like the Whale mode you have or the Shurflo) should not need a rebuild. The hose, even if not reinforced, should collapse unless you have really bad hose or have something that cause the suction to "pull it shut'. (Occasionally I have heard of that happenings with engine exhaust hoses with high temperature or overheating issues; and rarely with smaller hoses that were designed with "normal" wall thicknesses.) However, if you have a blockage, squeeze the hose because of the suction and plug-it up, maybe.

Backflush first. The Flush out downstream of the pump to the outside. I believe that should solve the problem.