Last weekend my TARC kids launched their rocket four times over the course of about 2 hours. The wind was moderate (5-8 mph) the whole time. The first two launches the rocket flew straight up. We were playing with the weight of the rocket to get the altitude. We were over the first flight, and under on the second. For the 3rd launch, we put in a weight that should have taken us between first two flights, but instead, the rocket veered hard as soon as it was off our (6 ft) rail, and flew well below our previous low flight. We made no change to the rocket other than adjusting the weight. We use BBs in a pouch positioned very close to the empty center of mass so that changing the weight shifts the CM by only tiny amounts. We chalked it up to a bad motor, and tried again with no changes. Pretty much the same thing happened, only slightly less severe. We flying Aerotech F42-8 motors on a 1 meter long rocket with a BT-80 body and four big fins. I'm not an experienced rocket person. Is this just the expected motor performance? Or should I look for a different problem?