Also have the A750 pump installed and it works very well, even after being left for extended periods.
It would seem that there are other issues here: as mentioned white powder suggests alloy corrosion, if this is finding its way into the fuel pump then one must suspect the tank is corroded or alloy components within the pump are corroding, either way a full investigation of the fuel system would be advisable
The A750 pump must is gravity fed and should be at least 4" below the tank outlet. The outlet and feed line should be a minimum of AN-8, preferably AN-10. If the pump is sticking after sitting, it would suggest that there is perhaps some corrosion, have you checked for water in your fuel? The pumps run on very close tolerances so they recommend a 10 micron full flow pre-filter to prevent any foreign objects getting to the pump and jamming the internals, which could also be an issue