Unfortunately if the unit is running a fifteen minute cycle it will point more to a problem in the duct design rather than some type of equipment problem. We have to go with the fact that the stat is on the middle floor and that is the only floor that the unit will listen to or care about the other floors can be considered slaves to the middle floor. If the system fails to condition the other floors satisfactorally, then you have a ducting issue which may or may not be as simple as balancing it out. More than likely you will need to have the ducting modified to bring the other floors to some type of balance with the middle floor.