i had a friend who's did something similar but would also chuck it out of gear completely in first and reverse. it had a code stored and 90% sure it needed a new clutch pack, if it was my car it would have been changed but he didn't change it in the end and sold the car (yeah one of those people) It's hard diagnosing something that can be one or two things, in your case not having any leads.. i would take it to a pro and get them to confirm what it is.