Definitely sounds like an issue with the secondary injectors. There are several inputs for the injectors to operate with RPM being one of them. When you say it happens at a certain RPM what does your throttle percentage look like. For instance you could rev your car to 6500 at 20% throttle and never engage the secondary injectors. This is why with light throttle you can rev past 5 grand, low load.

 

2 years bro....

Either he fixed it and never told us, or someone got a really good deal on an FD with a "blown motor"