You know I've gone thru several digital probe thermometers because of the same problems.
My husband is a HVAC-R tech an one of his tools is a $380.00 temperature gauge with 2 seperate leads. I use to test my probes with it and found that very often they were up to 14* off. I've bought the digital probes sold for reptiles as well as radio shack and wal-mart ones and found that they all registered differently. Even when I'd find one that gave correct readings it would not continue to give correct reading for long. I just figured it was my bad luck with thermometers.
I purchased a infared temp gun some months back so my hubby could have his tool back. I don't know how I did it before I gone the gun. Checking temps is now a breeze. Best of all it reads and displays proper temps an I don't have to deal with the probe lead wires in my viv's. Also with 8 to 9 viv's in the house at all times that's alot of money to have even the cheaper ones in each viv. With the gun I walk thru the house checking everyones temps an in just a matter of a minute or 2 I'm done.