DO NOT ADD MORE VENTS W?O FIXING THE LEAKAGE!! This will only draw more hot air into your ceiling.
Determining were it's leaking is given to a lot of variables. Is it T&G Ceiling? If so, was an air tight barrier (like poly) put above it? Is the fireplace built in? was the ceiling of the fireplace chase sealed air-tight (Fiberglass doesn't count)? Is the cathedral ceiling accessable to fix? Is it rafters, parallel cord truss or scissors truss?
If things are not done right the first time, sometimes it's difficult (Or expensive to solve).