I bought it about a year ago. It has about 27K miles. I suspected that the problem would involve one or more of the possibilities you mention...just did not know where to start. I have no idea what sort of maintenance the bike has had, but it has been taken care of if judged by appearance.
You can change over to a heavier fork oil/fork fluid and down the road think about adding the Race Tech Gold Valve Emulators to the forks. Check out the product at their website http://www.racetech.com/HTML_FILES/GL1800GV.html one side is a cartridge fork and the other is conventional.
Trike Lady has given you some good advice. I've heard Progressive springs mentioned as well from time to time. If you really have a lot of $$ some guys go for a full Traxxion suspension changover which for front and rear runs around $2,500. Too rich for my blood and for the number of miles I put on my wing..