I hate to say it, but I had a similar experience when I tested out Ranco and Johnson thermostats on my racks. I monitored 3 different racks for 2 weeks, one setup with a Ranco, one with a Johnson, and one with a Helix (as a control) and noticed temps swings of as much as 8 degrees on the Ranco and Johnson racks while the Helix rack remained accurate to within 1 degree. Unfortunately, I found no way to control the temperature swings that I saw and decided to stick with Helix thermostats. In almost 10 years, I've never had a problem with them.

Hope you have better luck than I did!

-adam