Had something similar with the wife's RRC... Fair play to her she got a bargain cos it was described as jerking and 'slipping' when warm, so she bought thinking it would 'just' need an autobox change...(like I wouldnt have enough to do)
She got it back and found all that was wrong was the fluid level was WAY too low..I topped it up and now running fine.
Start it cold, select neutral then check level dipstick. Don't bother doin it without the engine running or when hot cos youll get a false reading