TMC electric toilet impellor problems

[pohopetch]

We have a TMC electric macerator toilet that is about 4 years old. It has gone well for a long time, but the flush pump impellor disintegrated a few weeks ago. I replaced it with a new (NZ$65) Jabcso impellor which only lasted 1 week before also disintegrating (and jaming the motor). There does not appear to be any damage inside the pump cavity that could be causing this, but it is a bit slow to pull flush water through past the the anti-syphon bend (2.5 m all up from inlet). Does anyone have any idea which the impellor should be imploding? Is it simply because it is running dry for 5 or 10 seconds before the water flows?

Thanks
Peter