That sounds like it could be electrical to me.

I had something VERY similar happen once. I forget the terminoligy, but there's an electrical component that kicks in somewhere around 2,000 rpm. (I think it's called the stator). It basically doubles your spark to kick you on into the higher RPM power band. If this component goes out, you top out somewhere around 2,000 rpms with a load. If there's no load on the prop, it will go ahead and rev up to 6,000