A single grain of sand can stop an AC 110, or 500, in its tracks. I have had AC 500's jam up from sand so miniscule that I could barely see it when the motor was removed, and I also ran my units with pre-filters. Some minute particles of sand can and do get past pre-filters. Even course substrate will produce some fine dust like particles. The fact that you saw sand in the housing means that there is sand getting in. The 110 motor, impeller shaft, and impeller, are identical in the older units so that is not the issue. You either have sand issues (most logical), or got a bad run of manufacturer flaws. I suspect the former.