Had similar Problem with my RBI at beginning of the winter... I moved my rack closer to heater.. During coldest days (when my supper forget to heat up apartments enough I crank my herpstat to 107... i use sani-chips for substrate 1-1.5 inch layer) and control my temps 2 times a day with internal accurite in every tub and temp gun... coldest i have hot spot was 86.... cold side (front of the tube) 74.... ( usually have hot side between 89-91 and cold 81-79/78).

Sometimes when my supper really crank a heat i use spray bottle to raise humidity.. but doing it mostly when i see my snake is going to shed...

In small collection it works..... in bigger keeping steady temp in designated ''snake room'' is a must.