Hi Mate,
In regard's to the suspect sensor i would wait until it's reading correctly then unplug it and see if it come's up with the same negative temp reading, if so i would be looking at the sensor for dodgy solder joint's, the plug's & socket's for loose/dirty connections damaged wiring or inside the control unit for broken solder joints.
You could also short the sensor when it is reading incorrectly and correctly and see if you get the same temp readings. *** WARNING *** I doubt it but am not 100% sure that doing this will not have any adverse effects.
If the reading's are the same in the second test you can pretty much throw the sensor away... If the reading's are different then it's back to plug's, socket's or dodgy controller...
Hope this helps...
