Not wanting to get into any arguments, but my C90 runs smoothly through it's entire rev range. I find that using 2400rpm is a bit too close to max continuous. The law of diminshing returns also applies to my C90 in respect of speed gained against fuel consumption.
My suggestion would be to try various settings re RPM/IAS/LPH.
Don't forget that Jodels do cruise nose down in level flight.