MZ-S not rewinding roll completely

Ok...I've had this camera less than three months. I loved it, until ....

When I reach the end of the roll, the auto rewind kicks in, but only rewinds a couple frames. Then it stops. The LCD indicates film is completely rewound, but it fact it is not. This does not happen on every roll. Sporadically ...about 4 times in the last 16 rolls ....but I shoot weddings and cannot risk this in the middle of a ceremony. I am using the AA battery grip.

I changed the batteries, that isn't it. I've never mishandled the body. I purchased it from ABes of Maine, who only refunds up to 30 days.
Pentax wants me to pay $40 plus shipping in order to repair 2 wk turn around.

Anyone have this problem? Ideas?

Probably best to get it repaired if you are so dependent on it. $40 doesn't sound extortionate to me.