What do you mean a drop of 50rpm? If you're cruising or moving at all, a drop of 50 rpm is meaningless.
RPM is going to be a static number relative to engine speed. It's dependent on your gearing/tire size/speed. If your torque converter is locked which is often is at highway speeds, there is only going to be 1 rpm you can be at for a given speed. If you're over that rpm, then your transmission is slipping or your torque converter isn't locking.