Yeah, if you're using a jumpstart (on/off) type, then that could be the issue. You may want to invest in a herpstat..They are proportional and use a dimming feature so that it only ever runs the tape at the temp needed. This could help if you are getting temp spikes.

I have a couple jumpstarts and only use them for quarantine setups. I found that some worked better than others. I had one t-stat that I tossed because it had at least a 9 degree temp variance..It would allow the tape to get 9 degrees hotter than the set point before shutting off.