It's obvious that you know enough, by having a properly tuned, and working Late Ketland. It appears you may have tried bevel up, and down. Before you waste you time with a new frizzen, or try some of the offered advice, try calling Jim Chambers, and see what he advises. Not all Silers are assembled by Jim's crew, but he stands behind the locks, and may have you send it to him. Yeah that's right call the maker or supplier, before you do something radical that's given as advice on this site. Even the big guys like TC answer the phone, as well as Getz, Rayl, Rice, and the other custom barrel makers. Green Mountain's too busy making M-16 barrels. Pecatonica, and guy's like Dave Keck will answer your questions, and Track, Dixons, and Log Cabin will give you straight answers. It will save a lot of grief, and money. Just drilling a hole in a frizzen, and hardening it won't solve your problem necessarily, as you have to tune and fit it. Lightening springs, are not the answer either. 25- 35 shots may be the best you can get with that lock, but 50 to 100 seems more like what you should get. Sometimes you get a batch of soft flints. Also when Rich Pierce gets back to business, try his White Missouri flints.
Bill