About the thermistor:
Make sure the fin and the contact surface of the thermistor are clean.
Moving it UP on the fin will make the refer cooler. Down warmer. (It's a counter intuitive design)
Moving it further out from the central fin will make the refer cooler, in addition to up & down. I moved mine one fin to the right and got some improvement, but not enough.
Before you unplug the thermistor be sure to power off the refer.
If you unplug it (connector near the light) the refer will go into Basic Operating System (BOS) mode and cool according to time VS temp. It will get very cold & freeze your stuff so keep and eye on it.
If you take the thermistor completely out and let it come up to room temp (70F) it should have a resistance of somewhere around 9.5K Resistance goes down as temps go up, and it changes a lot with only a few degrees difference. When the refer is at 38F the resistance will be around 24K.
Sometime in the next month or two the makers of 'Snip-The-Tip' will be releasing and adjustable thermistor for the 1200 series. I built a protype from one of their Dometic units and have been able to stabilize my 1200 at 35F, or even colder if I wish.
If there's something else wrong with the refer, bad fans, low airflow, clogged boiler, bad control board etc. nothing you do to the thermistor is going to help.