OK, I understand.
I thiink Mondy really hit on the solutions to your isues here.
From my experiences over the entire range of Carve boards since about 1999, I would say that trying to sail you Carve 131 very near to it's maximum, with the mast foot forward of the center of the track is probably the central issue here.
I'm pretty sure Mondy would agree that sailing a Carve (any Carve) at near max. with the mast foot that far forward is going to do a couple of things, neither of them good, in my opinion.
First, the board is not going to be "free", as the mast foot forward is going to push too much of the board's hull into the water.
Secondly, having the mast foot that far forward is going to result in a "bound/rebound cycle that's very difficult (if not impossible) to control.
My guess (Mondy hit on this as well) is that somehow you are failing to commit your weight onto the harness or not doing it as soon as you get your front foot in the front strap and hooked in with your harness.
This lack of mast foot pressure (MFP) is what allows your board to bounce around so much when you gain a little speed.
The bouncing around affects your ability to steer with normal heel/toe pressure and you compensate by heading upwind which at least gives you a little better directional stability (when you don't have enough MFP).
So, and I realize it's going to be hard to break old habits, I would suggest you start moving the mast foot back a little at a time, try to shorten your harness lines (if they are well balanced now) and adjust your harness line position a bit to get the harness lines to the point where you can remove both hands momentarily and your rig justs stays in place, providing you with steady power.
If your lines are not balanced well, and about the right length, it's nearly impossible for you to relax, and just let the board accelerate.
The faster you go, the more out of balance everything becomes.
When you get things really well balanced, the line length correct for your size and stature, and the mast foot in the board's "sweet spot", things will be a little out of balance as you start to sheet in and rake the rig back, but the balance will improve as you sheet in more, rake back more, and gain more speed.
When you get up to full speed, with full MFP on the board, through the rig, the balance will be near perfect and you will soon have the board going at max. speed, all the time, comfortably.
If you can find a way, I'd really like to see the video, or even still shots of you sailing to check for obvious issues that might be fairly easy for you to correct once they've been identified for you.
Hope this helps,