Tub Temperature Fluctuation

So for those Freedom Breeder Rack users out there do you guys have problems with Temp fluctuations with your tubs in the beginning? I have a single level freedom breeder rack 10 levels high but I only use 4 of the tubs, one of the tubs is a temp tester. I have 11 inch heat tape insulated and I taped the probe inside the heat panel on the heat tape between the insulation and the heat tape. Thermostat I am using is a VE 300. I set this thermostat to 90 degrees F and the tub temps got to like 90 in the beginning but maxed to 100 degrees when I double checked it. I also had a Herpstat ND thermostat before and did the exact same thing 90 went to 100. So I don't think it is a thermostat issue because both thermostats are doing the exact same thing. Does anyone else have/had a problem like this? I have never used 11 inch heat tape for my previous racks so maybe due to the stronger wattage? Freedom breeders are a bit thicker tubs too so I do not know if the temps are holding better due to this thickness and the heat is just building up. Any advice much appreciated.