I am not sure if this is the same thing but your symptoms are identical to what I had a while back. I ended up replacing the entire clutch assembly, 2 speed assebly and rear pulleys ...but nothing. Start the car, hit the throttle and there would be this small movement ...then nothing. I have two RRR's and compared the two part by part and still the problem escaped me.
Finally I replaced the 1st gear housing (if it's called that
) and my problem went away. I think the bearing in there went bad or something ...still can't quite figure out what happened. Try it ....you never know.