Tires again (but this time on the tow vehicle)

I recently replaced the Original Equipment Michelin MXV4 H-rated tires on my
VW Passat 4 motion wagon (Variant for those of you in Europe) with Dunlop SP
40 A/S all weather radials. Other than lacking a speed rating, they are the
same dimensions as the originals. Although the new tires are okay in
regular driving, I've noticed that the towing performance is much worse.
The car and trailer combination (LS8 in a new Anschau Komet Euro Trailer)
used to be rock solid up to 75mph; any mild trailer wag would damp out
immediately. Now, trailer wag can set up at around 65mph, and it seems
less likely to damp out.

I'm assuming that one aspect of speed rated tires must be a stiffer sidewall
to handle cornering loads. I'm guessing that this may be a part of the
problem. Anyone else experience similar?

Erik Mann (P3)