Assuming that everything else is OK as you say, then it's possible that you might then have a sticking front caliper piston. They usually stick when the brakes have been used a few times, hot. To check, drive the car until it's occuring, then lift both front wheels off the ground with a floor jack and spin each wheel. The one with a bad caliper won't turn and that wheel will be very hot. The rotor usually discolors and warps from excessive heat so if you have a bad caliper, you'll probably need a new rotor also.