@Dcleonard I am with @PausS regarding PID tuning.... If the temperature swings way high, poaaibly way low, but eventually settles on the set-point temp, my guess is the PID tuning is off. I believe you can look at the Folger stock Marlin PID tuning parameters and use those as a starting point. Search Marlin Configuration.h for
You should see similar in your Configuration.h file.