Mike and Spark,
Unless I've drawn it wrong somewhere, I don't believe the comparator is set up to function at all as expected. I ran the sim and stepped the inverting input at 7 levels from 4.5V to 7.5V and got 4 different output states. I've attached the sim.
I have been over it several time looking for drawing errors, but can't find any. The symptom described could be from the changing resistance of the sensor in conjunction with the configuration.
I forgot to note the input Vs. output levels:
4.5V in = 12V out
5.0V in = 4.1V out
5.5V in = 8.75V out
6-7.5V = 0V out