I have the same problem,
A mechanic friend says it can be one of 6 things. 5 of them are really bad, the other is a problem with the shifter getting into the proper place (he said there is this bally thing at the bottom of the shifter that goes into these holes to lock it into gear and if the slot is worn it won't go in right or something??)
If anyone has had this problem and fixed it your input would be appreciated!