We seem to have found a solution to a problem similar to what you describe. We have 4 Lulzbot Mini printing exactly the same model. Since a couple of days, one of them shows random problems such as:
- Thermal Error, system stopped
- Bed temperature instability
- Print stops at some point during the print
Since we print a lot, we have to change PEI sheet very often. It is quite difficult to attach bed wires the way Lulzbot attach them and we don’t want to damage those small wires so we let them a bit loose. During print, those wires can sometimes be in contact we the side printer stepper motor. During last print we just changed a little bit the wire position so that they are never in contact with the stepper motor. The bed instability has totally disappeared.
I am suspecting that the side stepper motor is creating an inductive current that is transferred to bed wires when they are in contact. The random information is transferred to the bed wire (thermostat). The controller then reacts with the wrong information; sometimes it detects the wrong temperature, sometimes it triggers a controller shutdown and then stops printing.