Sorry to revive an old thread, but as it's only 9 months ago, maybe others are still battling this issue.
I thought I posted this earlier - but I turned the water pump cover to the opposite side, aftre cleaning
from it - can do this o some models and this cured the issue.
The over was worn slightly and I am pretty sure the resulting slight loss of effciency was enough that it could not overcome some operating conditions. Has been great ever since.