Go bananas how? Numbers, please. The temps are steady during the day at? At night the temps change to? Do the temps return during the day or do they stay off the mark?
One bit of advice, I can give you without hearing the answers to those questions is to relocate the probe outside the tub. Secure it between the mat the bottom of the tub. I realize he isn't dislodging it, but his laying on it may be affecting it in some way. Besides that, tape in a tub is safety hazard. Snakes have an uncanny ability of getting wrapped up in it. It can cause injuries.