I had a similar experience with a BA about 4 years ago. It would almost top out and then usually only ignite around 20% N1. We changed a bunch of parts finally fixed it a good while later after a few months of trouble shooting. But sorry can't remember what was actually changed that fixed the problem. I think that engine restricts and only introduces a very limited amount of fuel until around 20% where it then allows the selected amount fuel to be added. I think that was were the problem was, but I'm only a pilot with a bad memory so sorry not much help.