hi ralph
do you use a stablizer on the outfit
sounds like a stability problem to me the old dogs tail wagging the dog scenario
I had a similar problem when i first towed my bailey the dealer where I bought it said the alko ball hitch on the van meant a stablizer was not required but driving it down the motorway on the way home proved that statment false
having sold all my kit with the old van I looked for another and found one on e-bay an old scott type after fitting a new friction pad it work a treat no problems since then and iv'e been both ends of the country with it
a little tip here allthough you probably know this allready tighten it up so you can just move it and keep the nose weight at maximum for the car
I personnaly over do it by about 5kg to make sure there's enough down force on the ball which isn't a problem as the old scott has a rather heavy spring that needs about 35kg of pull to move it
also check the rear suspension of the citroen to make sure it's not too soft I know the picasso is used for towing quite a bit
so some one must have a answer for you
good luck colin