Sailor: 70kg, been sailing since last summer
Board: Starboard GO133, JP Slalom 42 fin
Sails: 7 to 8 sqm
Conditions: 5 to 15knots wind, flattish water
The wind picks up, board accelerates and front half of the board lifts out of the water. I'm positioned such that forward foot is between the front strap and mast, closer to the strap. The board feels tail heavy where I'm standing and the nose is too high. The rig feels right, can't stand any further forward to bring the nose down.
At this point, the nose will start to violently sway left and right 2-3 times per second. (It's similar to head shake/tank slapper when riding a motorcycle). It will usually put me off balance and ruin the momentum and slow the board down

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However, if a gust hits during the shake, the board will accelerate and jump out of the water, only last foot or two near the tail in the water. No more shaking, it's stable and fast

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The only way to get past the shake is to ride it out till the board has enough speed (not likely) or get some assistance from a gust.
Any suggestions as to what I'm doing wrong? Is it my setup, equipment or technique?