Pretty common complaint with the 5.5's.

Mine (5.5 Uni) has two sails, one was never a problem the other gave me fits.

What I did:
1. Invested in a high-quality, pre-stretched, small diameter halyard line.

2. Invested in a new halyard ring and tied like these guys mentioned...knot faces to the stern.

3. Had a sailmaker redo the headboard & top end of the bolt-rope to match the sail that worked.

4. Adjusted mast prebend to better match the sail luff curve.

End of problems. Worked for me anyway.

Good luck and sail fast.

Tom G