It's not uncommon, and something as small as a grain of sand coud be causing it - you'll just have to check it out. Don't be too surprised if you take it apart and don't see any cause for the stoppage for the same reason. If this turns out to be the case, while you have the pump apart I would at least fill the impeller well with vinegar and soak the impeller in vinegar for a few hours. If you may be getting precipitation from dosing in there (i.e. if you dose), the beginnings of the buildup you'd get are transparent when wet, so it's worth doing a strip down with vinegar like this when you see no other cause IMO.
Seems like lower-power (more efficient) pumps might be a little more susceptible to this than others (like the Mag series, just for example, which are a bit more piggish), so if your tank is not heat-bound and this issue remains a problem after cleaning the pump, you could try a stronger (higher power) pump if that's an option.
-Matt
