doesn't the power drop off at 5500rpm anyway on a 150?
I'd start by cleaning everything out. make sure airways and pipes haven't collected anything they shouldn't have. also pop a bit of cleaner in with the next fill up. that'll clean the injectors out.
if you've been driving it very hard, and it sounds like you have, then the DV could be playing up if you haven't already changed it for a forge one. if it is then it could be releasing presure when its' not suppose to.