Check for a short (most likely) or open wire to the thermistor. Make sure you're moving the X/Y/Z around as you may find it only happens with a bit of movement.
Re: A short. You may have a short between the hot end and one (or both) of the thermistor wires. If the wires are covered with PTFE or kapton, this may have melted/degraded over time and is now allowing things to short out. All it then takes is a bit of movement that "bumps" the exposed wires into something and hey presto, you get a "huge" temp spike recorded.
The firmware has limits built into it for this (which afaik will stop the print). You can turn them off, but I would be hesitant about doing that. If you do, watch the machine closely for the entire print. If it briefly temp spikes and then settles down again, it really points to an intermittent short. MAKE SURE to put the limits back into the firmware and load it back on your board, as it's there to protect your hot end, printer, house, and your life from danger.
Re: A short. You may have a short between the hot end and one (or both) of the thermistor wires. If the wires are covered with PTFE or kapton, this may have melted/degraded over time and is now allowing things to short out. All it then takes is a bit of movement that "bumps" the exposed wires into something and hey presto, you get a "huge" temp spike recorded.
The firmware has limits built into it for this (which afaik will stop the print). You can turn them off, but I would be hesitant about doing that. If you do, watch the machine closely for the entire print. If it briefly temp spikes and then settles down again, it really points to an intermittent short. MAKE SURE to put the limits back into the firmware and load it back on your board, as it's there to protect your hot end, printer, house, and your life from danger.