I've never had any problems with the standard set-up (2.5 V6 petrol Reflection). Biggest improvement is a roof deflector and a stabiliser set 50% above the recommended torque. Happily does 80 mph in France without any pitching or snaking.
Deflector improves normal mpg from 17 to 19 and makes for a more comfortable tow.
If the rear of the tow car is dropping significantly then the hitch weight is far too high.