Which Johnson Controls unit...

That's exactly what's happening. And changing to a different controller will not fix it. It (the big temp swing, which in my freezers used to cause the wort temp to swing more than I wanted) is the reason I use two controllers.

Controller #1: Controls average temp of freezer compartment, providing ambient environment a few degrees below intended fermentation temp. Probe is in the air in the freezer.
Controller#2: Controls the exact wort temp via a heat wrap. Probe is in a thermowell in the wort.

Yet you diagnosed it perfectly!

 

Sounds to me like the poster is using it for cold storage NOT fermentation...I could be wrong, but I think the answers are totally different depending on what you are using the chamber for.

 

Ah. Good catch. I'm reading on tiny mobile screen. For cold storage/lagering, I'd say just set the parameters to hit the *average* temp desired. It will take some time to measure results in order to determine the exact settings needed.