The only thing I can think is similar to what Krazie posted.
Binding of the impeller.
Have you taken the pump apart to clean it and reassemble it?
The other thought is the power supply, brown-outs and voltage surges (under current and over current) have blown many electronic devices, we think that power strip protect us, but I would beg to differ...
The best way I have found to really protect against power failure and surging is to run off of a battery...