T&T: CruiseAir Problem

[Cole Crockett]

Starting to use a little heat here on the lower Chesapeake (live aboard) 
and had a problem last night.
My aft ac/heat unit, instead of cycling the compressor off when the set 
temperature was reached it switched to a/c then back to heat when the 
temp dropped, good for drying out the boat but a bit of a waste of 
electricity :). Probably a little tough of the reversing valves to flip 
while the compressor is running too.
The controls are the old 3 knob style, I am guessing something is stuck 
in thermostat? Anyone ever seen this before?
Thanks!
Cole