Mijostin's comments make sense. If the arm is already too heavy (meant for a low compliance cartridge) there is nothing much that can help. You have a basic incompatibility, and perhaps a beef with the dealer who sold you the cartridge. It is not the fault of the cartridge--it is like putting the wrong kind of tires on a car and blaming either the car maker or the tire maker for the poor performance.
While you might not be able to cure the problems that result from this incompatibility, you might be able to reduce the feedback by changing the isolation and tuning of the table's suspension (try different platforms, different feet, try making the platform the table is sitting on more rigid, etc. If your unipivot has provisions for damping its motion you can change the amount of damping.