could be your thermostat. what kind is it and where did u place the probe? substrate shouldn't really matter. a snake will push it aside or get under it if it wanted to. the range of temps bouncing between 83-93degrees is crazy. often there's a 1-3degree leeway, but a 10degree one is kinda unusual. at the extreme end, it's a little hot. u don't want your BP's sitting on a hotspot over 95 for an extended period as it will get burned. maybe other forumers will chime in to help troubleshoot your heat tape issue.

also QT all sick animals. why risk it? and 4200grams?! wow!! whatta big girl!