Yeah, I adjusted mine multiple times without any luck. I had extended the shifter rod, as mentioned on this board, as well as some videos I watched, and that resolved the shifting issue. Now, after it slips into forward or Reverse, it never slips out.
PS: If you go this route, remember to readjust the cable to compensate for the extended shifter. I had to do the adjustment an couple times after extending the shifter and then the problem was resolved.