How to decrease sensitivity to heat-bed temperature? Sometimes I use a DIY 3D printer running Marlin firmware and I have a hard time to set my heat bed temperature. when I set it to 70&nbsp;&deg;C for PLA, after a few minutes it decreases to 67&nbsp;&deg;C and I see these error:

READ: Error:Thermal Runaway, system stopped! Heater_ID: bed
READ: Error:Printer halted. kill() called!


Since then the communication with printer is lost, the printing process stops and I have to reconnect to serial port... It's a disaster. I guess I need to lower the sensitivity to 3 degrees at least. I don't know how!?
I had a similar issue when printing with ABS, because my print cooling fan only activated once it got to a certain height above the bed. I'd say you need to do a PID tuning session, insulate the bottom of the bed better, and see if you can make sure your cooling fan doesn't blow air over the bed itself.