Derailed, not sure I am in the position to offer advise, am still learning. Found the same result you commented on, as I progressed to speed increase and pointing that bit higher.
If your on a wave board, fin movement forward or back. Forward for control, back for speed. (I think that’s how it works)
Slalom board, check fin is in flush with fin box, and not sticking out a tad, disrupting flow and allowing turbulence to start across fin profile.
First time it did it for me I wondered what happened, fin came off/loose or something. turned board over and checked, na all OK. Figured out that this is normal if I get stance/pointing a tad wrong. buggerised around a bit trying and adjusting til I could feel it as it started, then dragged rear leg towards body to kick nose down wind a bit and it stops sliding away.
Crew like Decrepit been around a bit so will have better idea on what the issue is than me. Keep experimenting, you will get it to a controlled point it wont be a pain in the arse.


Now all we have to do is wait til the squashed air comes back