digital aquatics temp probe issue

[e46er]

my digital aquatics temp probe sems to be acting up.
yesterday morning it read around 68 degrees 2 hrs later it was reading 87 degrees now this morning its back to around 69 degrees all while the tank is maintaining 78* with a different thermometer.

it appears clean but whats the best way to clean it or is it toast?? its around 2 years old or so.......