just to review this.....

do you see a rapid (within 30-60 seconds or so) elevation to the extra 2000 rpm, or does it slowly gain rpm through the run ??

If you throttle back to low/mid throttle..... hold it there for a minute, and then go back to full throttle..... does the engine jump to where it was, or does it drop in rpm and then starts the RPM climb again?