Wozer

the fact that it is on its own plug means nothing....unless by some very small chance that plug is the only one connected to the breaker...when I had my house built 7 years ago I had one plug installed with a dedicated breaker for recording use...and I paid extra for that...

the point is that its what is connected to the other plugs on the breaker that most likely matters....or perhaps the breaker is starting to go...I did a quick Google search for ac units with the specs you vaguely list and it appears as if they all fall within the ratings of a 20 amp breaker...but know this, the biggest draw that unit will give is just when it starts up, and the load it pulls may be towards the limits of the breaker just by itself...if you have another high amperage piece drawing electricity at the same time, then that is most likely the real cause...next time it throws a breaker figure out by deduction what other items are also on that circuit and deduce what item is also drawing a lot of amps....