Maggie -
A couple of things suggest themselves to me:
1) Try blow drying (low temp setting) both the sensor probe and the connector pins on the baffle immediately after washing.
2) Check the connector pins on the baffle. Do they look at all corroded? I would get your water tested to see what's in it, and then use a very gentle cleaner as well as maybe a pH neutral bottled water (just on the connector) to see if that solves the problems. I would also be very careful about getting water into the holes where the baffle connector pins make contact. They may be corroding, too.
An external thermometer is not the answer here as the machine is not displaying the right temperatures. You'd need to run the machine completely manually, which means -- for the quantity of chocolate you mention -- that it'd be faster and easier to hand-temper. You should probably know how to hand-temper anyway if you don't already because that will help you diagnose what is going wrong.
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@DiscoverChoc