Are you placing the ribs above or below the temperature probe? The only reason I ask is that the meat will interfere with the flow of heat to the probe if placed below. In the above position, cold air sinks, the opposite of heat rising and can have a similar effect.
If you look at a conventional oven be it gas or electric, the probe or probes are up at the top, never in the middle. Now look at the probe place on other makes of smokers. They are all pretty much in the same location, for a somewhat even cycling of the element within a certain temperature tolerance.
I am sure Steve will address your issue. You might also want to run it up empty at 250 and see what you get after an hour or so. But let me say one thing: It can be maddening to expect dead on accuracy between an analog controller and a digital probe. I would save the digital for cooking to temperature and as long as the food is good, relax and enjoy. As smokeasaurus said, "they don't call it lo and slo for nothing."
