I also own a used SM150 and love it. Mine had a lot of miles on it and I gave it a decent cleaning. I used to own a used SM008 and I had irregular results sort of like what you describe. I called Cookshack and got a new power cord, new vent nozzle and some stainless steel shelves. That made all the difference, I can't say it was the old brittle power cord or that the old vent was the culprit (it was really bunged up) but after that I got consistent and good results. It prompted me to look around and that's when I found my SM150. AFTER a decent cleaning and some new shelves it's the bomb. Haven't had to replace the vent nozzle yet but for the small money spent it would be a good thing to do. I have read repeatedly to not put food near that temp sensor/probe. If I do put anything on that shelf it's in the front section way away from the probe.
So worst case, maybe buy a new vent nozzle, a probe and put that new element you have in. Make doggone sure your drain hole is always free, lining the bottom of the smoker with fresh foil ever time is easy as pie. Your running the smoker empty for a bit is pretty smart. I home roast my own coffees with a fancy thing called a Behmor 1600, and they suggest a cleaning cycle every 5 roasts - that means running it like I'md doing 1/2 pound of beans but empty, it burns off all the oils that accumulate on the basket and heat lamps - it really improves the consistency of my roasted beans.
Give Cookshack a call, they are really a great and very helpful bunch. You got yourself a heck of a unit and in no time you'll have your questions answered and your problems solved.