The root cause could be a number of issues like; lacking proper tongue weight, WDH not adjusted correctly, TT not towing level, TT tires under inflated, etc.. I will assume that you are using a standard friction control sway bar and it's working properly.
For the 25Z the 1,200lb rated WDH is more then enough hitch when you consider the 25Z has a UVW of 4,700lbs and a GVWR of 6,000lbs. The loaded tongue weight should be 13% to 15% of the loaded TT weight on a 28ft long TT. For example; if the 25Z loaded weighed 6,000lbs (GVWR), then the recommended loaded tongue weight would be 700lbs to 900lbs........, thus the 1,200lb rated WDH is more then enough.
IMO a visit to a CAT scale will tell you a lot (if WDH adjusted correctly, TT loaded tongue weight adequate, TV/TT weight limit check, etc.). The following JOF link is a TV/TT CAT scale weigh-in "How To": http://www.jaycoowners.com/showthrea...igh-Your-tt-tv
In most cases the root cause of frequent TT sway is improper TV/TT weight distribution, and/or WDH adjustment. There are other possibilities, but I would in the process of elimination perform the CAT scale visit first.
If you can, post a close-up side view pic of your WDH with your TV/TT hitched...., sometimes the root cause can be obvious.
Hope this helps.