Dunno if this helps, but I've got a faucet in my kitchen that was
exhibiting similar behavior. The problem turned out to be a bad design,
where hot water caused damage to a vinyl part in the sprayhead.
A call to the mfr revealed that they had discontinued production of
that model sprayhead(wonder why), but they offered a free replacement
to the new model.
I'd send off an email to Moen, or give them a call. If you are unhappy
with the way your sprayhead is functioning - I'd bet a new one will be
in the mail shortly.