I haven't had many opportunities to fly my Mavic lately, but the last few times I've flown it, it was kind of weaving and bobbing around instead of hovering/flying steadily. At first I assumed it was due to flying in a moderate wind (in late May), but the last two flights the wind speed was much lower. During today's flight the wind speed was only about 7 mph according to the nearest weather station.

What's going on? My previous flights in early May were very smooth and steady.

Normally if it won't hold steady it may require both a compass calibration and an IMU calibration.

Like Jenee mentioned, please try to calibrate the IMU and compass. Besides, make sure the GPS signal is strong enough to fly, more than 10 satellites can be detected.