My Prusa i3 clone was working fine with Marlin 1.1.8. I had the ordinary red PCB MK2a bed.
Then I switched to RepRap mendel Heatbed MK3 aluminum plate 3mm. Now I get the thermal runaway error all the time and the temperature is never reached. I have checked the voltage, NTC and the temp all is correct.
I have tried longer time and hysterized without results in the configuartion_adv.h
I think it need more time to reach the temperature and before the temp security starts.
If I restart all the time and after several restarts reached the temp 110 C degree den everything works nice with a perfect temp over several hours.