Your super skids are definitely adding to the weight, and perhaps contributing to your head bogging problem. Maybe the autorotation bearing, too. What are your pitch and throttle curves? Obviously you aren't getting enough head speed.

I suspect your throttle curve should look something like 0-30-65-85-100. Pitch something like 40-65-83-93-100. I don't actually have a CP with a DX7 with which to test them. When you verify these with a pitch gauge, are the measured pitches correct?