Strange thermal runaway problem
I've been using my MK2S for years, and have never had this issue. I start a print, then consistently around the 3rd layer or thereabouts, the hotend would shut itself off (goes to 0), then the temperature would gradually make its way down from 240 (I'm printing with PETG) until I get the thermal runaway error and the printer would stop, and temperature would continue to go down until it reaches room temperature. The screen would read 70/70 bed temp, and 22/0 for hotend (once it reaches target). It just started happening yesterday out of the blue. This doesn't appear to be in any way related to the thermistor or heating element, because if I were to pause the print right before it reaches the 'thermal runaway' message, I can manually change the temperature back to 240 and it would go back to 240 without issue, then resume printing. When I do resume, the temperature either behaves as expected until the end of the print, or change back to 0 and the temp would start to drop again. Just a few minutes ago, I preheated the hotend to 215, then used the "Load filament" option to load the filament. It loaded, then asked me if filament is clear, and I said yes, then the temp just set itself to 0 for no apparent reason. I've owned (7) 3D printers so far so I'm quite experienced, but this issue has got me puzzled.