A new wafer usually won't change anything, it's the mechanical switch that the wafer presses against in most cases. you might check that the wafer is expanding evenly. remove it from the screw, run tap water over it and adjust the water slowly from cold to hot and watch how it expands.
you can also check that when the adjustments screw is turned (with the wing nut loose so it can turn freely) that the wafer doesn't wobble excessively.
the Hova bators are usually pretty good at holding temp with minimal swing, i have two i use as hatchers and they are rock solid.
anywho, headed out for the night, i'll check back in when i return home and feel free to ask any questions you'd like, that's what the group is for!!