The breaker to our AC keeps shutting off. No set time that it runs before doing this. Sometimes 15 minutes, others several hours.
I have changed out the breaker already. AC checked for freon , that was OK and unit was cleaned. Does blow cold when on.
What is the next step?

Do you have 1 breaker for all ac, or a breaker for inside Air Handler and 1 for Outside Compressor Unit?
Could be Compressor, An Amprobe may show too much current from inside or outside fan, or compressor.

Does breaker meat minimum size stated by compressor manufacturer.

Yes 40 amp breaker is new

Follows Stratmando's advice, have someone check the amperage. I'd bet that the condenser fan or compressor is drawing excessive amperage... how old is the unit? Motors which start to fail often heat up and begin to draw excessive amperage. The compressor is more likely the culprit here, but an amprobe test on individual components will verify. Good luck