Your freezestat should be an averaging element type safety. If any 12" section gets to setpoint, you trip. Pretty easy to test with a rag soaked in ice water (remember, you're not building a watch). It needs to be within a couple of degrees or so, with a setpoint of 36. Lots of good advice already given here about things to look at, also. The freezestat may be defective, but as already mentioned, it's more than likely just doing its job. You'll have to look at it and from an operational sequence standpoint, decide what could be causing it to trip. Tripping on the off cycle? No 100% shutoff damper shuts off 100%, and cold air likes to fall to the low point and force the warm air out. Running? OA/RA dampers not operating properly, low limit controls not functioning as needed. Bazillion different answers to this one, and a lot of them depend on the setup of your system. Look at the situation as a puzzle and figure out how the pieces go together. Good luck!