Fluctuating temperatures while driving

you did all the stuff i would have.......possibly radiator or head gasket..issue.....don;t just go and pull the heads...you need to air up the cylinders on tdc and remove rad cap and look for bubbles....or you could pull all 8 plugs and pressurize coolant system overnite and see if it fills up plug holes or loses pressure...only go to 15lbs or you will pop heater core/amongst other stuff I hope this helps....if you need the tools..if you pay shipping I will send you what you need to check it..of course I will need back when you are finished.....PM ME