Pictures whilst sailing would help. Can you get someone on a rib to take some photos from various angle when sailing to windward. It is important to get mast set up to the luff of the sail as most lift is generated in this area. With an old dacron sail the boltrope may well have shrunk and require a degree of downhaul just to achieve a base setting.
The 5.7 is boomless and does not have the adjustable clew traveler of other Nacras right? Which hole on the clewboard was being used? What were the conditions? How tired is the sail?
Best bet is get a local sailmaker or experienced catsailor to check it out. You could spend an age changing one thing after another when expert help could have it sorted quickly. There is nothing so annoying as a cat not performing.

Cheshirecatman