Unstable Hotend temps

Ok I have changed the heater cartridge and the thermistor on my Monoprice Select Mini V1 and did a PID autotune 3 times and my hotend still isn’t stabilising heat wise. I set it to 210 and it will up to and sometime 5c over then drop to at the lowest it’s dropped 196c then back up to target then drops again. Is my board going? Still prints ok unless i get a sensor fail cause it drops to much and doesn’t come back up fast enough its become unreliable to leave over night.