Good Afternoon,

I am in a troubling situation and not sure where to go next. I bought a 1998 3.0 Ranger with 168k miles on it. The cooling system is very odd, I have pressure-tested the cooling system and it only dropped 1 psi in over 35 minutes of testing. I have done a leak-down test and it passes along with a compression test where all cylinders test 120 psi or very close to it. I have replaced the thermostat, lower radiator hose, and flushed the cooling system. When I burp the system I can drive forever and the temp gauge never does anything odd and the heat constantly works. I leave the truck sit overnight and go to drive it the next day and the temp gauge will drop and then come up and then drop and come up again, etc. The heat will work and stop working as the temp gauge goes up and down and it pushes the coolant into the reservoir tank, could I still have an air bubble in the system?