Steve hit it with the a/c. Have an HVAC guy take a look at it. A bad breaker would trip. Don't put in a larger breaker. Sounds like your compressor or a fan motor is going bad and is pulling a lot of amps. Could also be a bad hard start capacitor.
If you have a voltmeter, check to see what your total system measures. You could also be approaching brownout conditions and the total voltage to yours and everybody elses' houses could be way down.