You're probably right about the bator not holding temp steady, but it might be a design problem with the bator having a thermostat that is set with wide tolerances rather than a sign it's about to break down. I don't actually know, but I'm assuming the more expensive bators might go on/off if the temp varies by a tenth of a degree and the cheap ones might not be as sensitive, maybe only going on/off if the temp varies by a whole degree, maybe more.
Anyway, if the air temp fluctuates up and down quickly, not slowly, then there's every chance the internal egg temp is staying fairly steady. The wiggler will show if that's what's happening or not. For $2, it's much cheaper than just assuming the bator's rubbish and buying a new one. I'd love to put a wiggler and a SpotCheck in my bator. When I open my bator, the temp drops right down to 95 straight away, and comes back up again within a minute of closing the door. I bet the eggs don't even register it!
Out of interest, GotGame and Beesong, what bators are you using?