120V x 15A is 1800watts. You're right at the top of what a 15amp breaker will handle. If you're running computers on the same circuit, this will continue to happen. The best thing you can do is move the iron to it's own circuit, or one that's not managing computers and such as well. I do think you'd be best with a 20A circuit at the least though too. Running all the time at the peak of what the wiring is capable of can certainly cause some long term issues.