Issue with weird heated bed temperature fluctuations
It really looks like either a bad thermistor or bad electrical joints. Are the connections to the thermistor itself crimped, or soldered? Are there connectors near the thermistor that can get heated up by the bed?

Electrical connections that are mechanical in nature (such as crimped, or using a connector) can degrade quickly in heated situations, particularly when the heating is intermittent.  Consider replacing crimped connections with soldered joints where possible, and where not possible use gold plated connectors and crimps to avoid oxidation issues. Oxidation and poor mechanical connections will change resistance based on movement, giving you apparent increases or decreases in sensor readings even when the sensor output is unchanged.

Chances are good that the thermistor itself is fine, but that the connection nearest the thermistor, between the wires and the thermistor, is faulty. If it's crimped, you may be able to simply add flux, then solder it to increase the bonding between the wires and the thermistor and decrease future issues with oxidation.

Note that soldering is really only useful for the thermistor on the bed.  The head thermistor must be crimped, as the head temperatures can exceed solder temperatures depending on the target temperature and the solder alloy.