Heater gets cold temp. gauge rises because there is air in the system, had pressure tester at 50 lbs for 20 minutes (no leaks) any idea's?

Did you replace the thermostat or some other component related to the cooling system ?

Thermostat was replaced by a mechanic that has been in business for over 25 years, should be capable, also i had him replace because of this problem, when he replaced it he told me that it was an okay unit, did not need to replace, still having problem he was right.

Are you sure the fans are coming on ?

Yes fan is working properly. I would just like to know (how) air is getting in system, the pressure check showed no leaks.

Robert did u ever figure out this issue, im having the same