Possibly your stems are getting some cabinet metal temp? For the 8 racks, I used a remote probe poked thru one of the ends of ribs on the right side 2nd shelf down, yes 2/3 of the probe was exposed. The remote digital read almost the same temp as the MES controller. I'm only guessing about your about your stem thermo. though based on an experience last year where a probe stuck thru a potato, had the tip against the inside cabinet wall, and the cabinet was a lot hotter than the MES controller read out, like 70+ deg.
I also observed that recovery after 4 or 5 hours of smoking is very fast compared to recovery after 1 hour of smoking.
Thus I now preheat the smoker for over an hour, believing that all the metal cabinet/metal racks/pan/ etc, completely heated will help get the MES to cooking temp faster after you load your meat. However on the 4th it took 3 hours anyway. I used the top 3 shelves with 3 ribs per shelf using rib-racks to stand on sides, and 2 laying flat on one shelf. I think from memory I put the two flat ribs on the bottom 3rd shelf, (can't rem for sure). If I did, that may explain why the MES took so long to get up to temp with those 2 flat ribs blocking the heat. However the MES sensor is near that shelf so I don't know.
re: the 8 racks of ribs for the 4th. As I said it took 3 hours to get to temp of 225. However I had the controller set to 270 to make sure the heat element stayed on till internal cabinet temp reached 225 (measured by the remote probe). I opened the hatch to check for pull back about 30 min to 45 min. after reaching 225. To my surprise some of the racks had almost 1/2" of pull back (I prefer about 1/4 to 3/8" then foil). So obviously the contineous heat was cooking the ribs. BUT AT WHAT TEMP?
On Monday I did a 8.5 lb pork shoulder/butt, after 1 hour preheat, I started the smoke about 3 am with outdoor ambient at about 65-70 F. The MES was at temp 230 in about 30 minutes.
Load is definetly a factor in the MES coming up to set temp, I have done 8 and 10 racks of ribs before and temp always took a 2+ hours to come to temp.
Another question is what temp should the MES be set at when your waiting for the MES to reach your desired cook temp. If I'm there to tend the MES I will set the temp to 270 and reduce to the usual desired temps of 220, 225, 230, etc, when the digital control or an remote probe shows the internal temp has achieve my desired cooking temp. The thought here is to keep the element continously on until cooking temp is achieved. However this may be a problem on items that don't need long cooks or on a big load, actually over cooking whatever is being smoked.
What are other MES owners especially 40" doing for temp control, in the time from initial cook start and when internal cabinet temp finally achieves your desired cooking temp?