Maybe you could shim the rear wing for high speed and see what happens ?

If I recall correctly from Berowne's videos which helped me heaps with my speed. Long lines - always loaded, mast foot back and heel the board. Once you're loading the sail for power it generates downforce through the mast foot which needs to be countered by back foot pressure otherwise you're touching down and doing the dolphin up-down motion. So the equilibrium is achieved between these two in addition to your front foot. essentially the sail becomes your instrument of downforce.

WillyWind said..
For the front foot at low speeds: Maybe the front wing is positioned too much forward so it tries to take off even though there is not enough board speed for it. Try moving your feet/straps forward or the wing/foil back.

+1. When the trim is wrong, it sometimes results in behavior that is quite unexpected, and does not seem to make sense.

I sometimes have situations that feel similar to what you describe. It usually happens when the wind picks up suddenly and a lot, and I end up very overpowered. Then, the foil often wants to pop up right away, and feels unstable until I manage to push down a lot with the front foot, and pick up some more speed. Switching to smaller gear, or at least adding shims to reduce lift, usually fixes this.

If trim changes significantly with speed it's almost always down to shimming. Usually the tail, in this case I'd shim for more down pressure from the tail.

I can't comment from personal experience as i don't have any to that specific foil, but found a review that speaks a little about the matter:

www.windfoilen.nl/en/f4-foils-freeride-windfoil-review/

from the link:

F4 Foils mast extra attention for trim As with the F4 Foils Freerace , we had to adjust the angle of the mast slightly to ensure that the nose of our board stayed up nicely. The angle of the fuselage to the bottom of the board is standard 0 degrees. We may easily get on the foil, but during wind foiling we have to tilt our sail quite far back to ensure that the nose of our board remains neatly horizontal in the air. In addition, the board slows down considerably as soon as we have a touch down.The solution is to use special spacers or shims to adjust the angle of the mast in the board. After some trial and error we arrive at an ideal angle of between 2 and 3 degrees. With this angle we have more lift and the nose of the board stays nicely in the air. We feel a better balance in the complete set and that also gives a little more power on average. We stay also more comfortable, which also gives us more control. As an additional advantage, the nose of the board rises nicely again when we have a touch down. We also give the back wing with the included shims a slightly more positive angle to get a little more lift. These 2 adjustments ensure that we have sufficient lift at all times and have a good balance in the set.

so maybe same thing going on for you?

That's weird, almost like the tail is stalled or doesn't have enough angle for low speed.

I've lost control like that sometimes when getting caught in weeds wrapped in the foil, but that caused me to not be able to fix it and then crash, it wasn't uniform up and down issues but also roll.

I think you'll want more stab power.