Wow, this is great info, I can’t thank you enough!
Temperature tower
I printed your suggested temperature tower - see photo. I verified that the gcode set the temperatures correctly at each level and also confirmed on the display as it went along. The quality is not great at any level and the print failed at the 215 level (which as you know was my default print temperature before). I agree that 230 looks the best, but it’s still horribly stringy. When I was printing it, the print head would collide slightly with the top of the print at any level as it printed the infill. This happens all the time with my printer.
Thermal runaway
I’m probably wrong about this - the reason I thought this is because when I disassembled my print head to clean it out, the nozzle appeared to be mostly empty of filament, but I couldn’t remove the filament because it was too wide just below the feed mechanism, thus I thought it had begun to soften before the hotend. I read your suggestions and there is no evidence that the thermistor is faulty.
Jams
Yes, using 2.85/3mm filament. Tension is good. I cleaned the hobb gear right before this print by fully disassembling the print head. Esteps is 420. From another post of yours and this one, I copied your retract settings and that seems a lot more likely to be the issue so I will report back.
Slicer Settings
Ah, I have been looking for IC3D PLA profiles but have not found any so I’m using PolyLite! And yikes, the print settings for PolyLite in Cura show a default printing temperature of just 205C. These are the stock settings from Cura LulzBot 3.6.20. I will use a build plate temp of 55C from now on. I will buy a set of calipers.
I’ll try a calibration cube next. I was wondering if my temperature tower would be my first successful print at least to the 215 level, but no!