Unstable Extruder Temp

Very unstable extruder temp, happens only when heating, only sometimes when cooling down but never during a print when the temp is constant. This started about a month ago but only happened every 10 prints or so. I tried PID tuning ,different thermistor pin and recrimped the connector. this also cause a "Heater extruder not heating at expected rate" Using a Revo hotend.