[MrBonus]

I have the Rennshifter and Stomski coupler on the exact same transmission. I spent a good bit of time in the past week adjusting it after it came loose and still wasn't happy. It turns out my clutch was out of adjustment. An hour of up-and-down on the lift and the car shifts so much smoother after getting the clutch dialed in.

You also should check the condition of your clutch cable and pedal assembly if the above doesn't work.