I forgot to add that the tuning should be done with a half empty fuel tank.
Without actually being there to see and hear how your engine is running, I would definitely look at your clutch again. Check each shoe against the others for rebound strength and see if there is a weak spring. Are you running composite or alloy shoes? Alloy will usually give a bit better punch as they usually allow the motor to spool up a bit more before grabbing. This helps to alleviate some instances of a low bog.
If your clutch is all in order, try richening the HSN 1/8 turn at a time and see if it helps. It normally wouldn't make sense but the Go carb is a bit different beast.
Here is a link to some great info posted by MassiveMods on the subject. His post is about mid-way down the page. It helped me a lot when I first got my Go engine.
http://www.rctech.net/forum/nitro-of...hread-131.html