Hey Sheartip,
Once this starts happening, especially with sails that are 4+ years old, it's time to either upgrade or just fix with normal waterproof gaffer tape. OK you fix one panel, then another panel will split so it's off to the repair shop, then something else goes, and it all ends up more expensive than just getting a reasonable 1 year old sail... Don't know what it is about monofilm but once it starts splitting that's the use by date gone.
I just tape the lot up with gaffer tape, doesn't look that flash but lets me save up for that next new sail