Tighten it more
The only thing you should have to tighten is the wing nut. If hand tightening does not hold it securely enough, you may have to put a tool on it. Looking at my 94, I don't think you can overtighten it, the red gasket inserts should give before your rudder does. The owners manual says: "Do not cleat the pull down line in the down position. Use the tensioning clamp for this purpose. If you run aground the rudder should be allowed to ride up against the friction of the clamp and not be retained down by the pull down rope".That indicates to me that the clamp and gasket material were designed to do the job.You might check the red rubber insert to make sure that it is all there, and that it has not hardened so much as to not have any give. If that is the case, it is replacement time. Just don't be tempted to tie the rudder down permanently, that could give you a lot more problems than a hard tiller.I have been using the "tighten it down hard" method for over 9 years, and I know of at least one '95 here in town that does the same thing.