RX75 bucking in forward

my 20 season old rx75 is bucking badly in forward. It is a transmission/drive issue, engine runs smoothly. Blade drive is fine. Almost acts like it is in the differential but hard to tell. Belts are relatively new and worked fine when i first installed them. Any thoughts.

if it is a gear drive type trans and is jumping it is usually a sign that the shift keys are worn and are slipping in the corresponding gear.