I had a pool guy come out and take a look at my mess of a pool (Bad motor --> cavitation damage = useless wet end?), and he was rather stumped as to why I keep blowing up motors and having problems. He did note that there was what seemed to be a lot of pressure at the wet end (25 psi) versus what was at the filter (18 psi.) He seemed to think that could be part of the problem, or that there was the possibility of the (single) return line being plugged up, making the pump have to work much harder.
Does any of that sound plausible?