Mikey,
Let me get this straight: You cooked 3 racks of baby backs for 5.5 hours, and they turned out good. You say they could have used a little more. How much more? Were they tender and moist, or not fully-cooked? I usually take 5.75+ hours to cook 3 racks of baby backs, so your timeline is about right.
Your problem is most likely probe placement, not the smoker itself. If it was really only getting up to the low temps you indicated, your ribs would NOT have been edible at 5.5 hours! Before going into panic-mode, I think you need to evaluate your temp probe placement, and try different locations (like below the meat).
Not trying to diminish your concerns, but we've seen this issue before. The results of the smoke are great, but the analysis of the smoke may be faulty. It seems, from your posts, that everything was going totally wrong, yet the ribs were good. ??? And then, when you removed the meat, the smoker magically shot back up to temp? This is probe placement, plain and simple. I saw this issue while testing the digital model 2; moving the probe (Maverick) during a smoke would make it vary as much as 20 degrees!
Maybe you smoke something like a Boston butt, and leave the box probe out (I rarely use mine). Put the meat probe in, set your temp to 225, and let your SI do its thing. If, in a reasonable amount of time (1-2 hours per pound) your meat is not done, you'll know you have a problem with the smoker, not instrumentation problems.