Schuman

The secondary one I used I have checked with the ice water but not boiling and it was within 2 degrees (no way to calibrate that I know of) but the stock thermostat one runs off an algorithm to estimate the temp in the center of the grill so I can't do much with it

Yes. I sealed the lid with gasket, and the top of charcoal hopper with gasket and high temp sealant but the air could easily blow in the good size air vent

I did about a 12hr cook yesterday and it was way off for probably 7hrs, it wad an extremely windy day and only about 40F outside so that may have factored in. Once the rain clears I'm going to run it with nothing it in except some temp probes and grate thermometers to see if it was maybe a fluke, I may have one with some faulty software or something. I don't believe it was opening the door as it was off before and after door opened and I wasn't opening often untill the end but it's always a possibility

I figured it was a few months old, wasn't sure if he was active or anything anymore. Was worth a shot

Is your temp controller thst accurate in cold weather too? I havnt used mine in anything over about 45 F but the temp controll a d the actual temp readings from the probe that came with grill and z separate temp unit both show actual temp about 30 to 40 F lower than set