I just installed the polaris booster - and noticed a couple of things, these may / may not be happening with your setup
1) the pump was cavitating like crazy - sounded like it was full of ball bearings, and had a steady stream of bubbles blowing into the pool (and the nipples leaked), (tightened all of the nipples to wrench tight, and then the cavitation stopped)
2) the Polaris was able to run without cavitation, pretty much just on the water being sucked thru the returns (they are 3/4 or 5/8 inch returns), (it pulled about ~0.5lbs of pressure thru the filter without the main pump engaged)
3) Do you have a heat or water issue with the location of the pump? (Maybe something is causing the motor to overheat or maybe something is leaking into it? )
i don't think that the motor on these things would die unless there was some kind of problem with the shaft (I ran a whisperflow dry
12 hours a day after I closed the pool in october for about 3 weeks, and it worked until I just replaced it (this was 3-4 years ago))
Did you try taking the motor off and turning the shaft by hand to see what it felt like? (I'm not a motor head - but it might tell you if there is something going on with the motor, tightness or binding / grinding noise when you turn the shaft.)
--Adrian