Got to be careful with those impacts. Some are gutless wonders and some like my AirCat can literally snap frozen bolts in two. I snap a few of my impact extensions in two especially when using cheap Chinese ones. That what I get for 1300 ft-lbs of break away torque. Even the gutless wonders may not be running at full power due to airline restrictions.
As for belts falling off double check that all the belt retainers are in place. Many home DYIers leave these off as they feel they are unnecessary. Even shops do it sometimes as they have inexperienced techs.
