What kind of wheels you on? I only got them when I was on a HED H3C up front. Scary yet funny when they happened. Learned to just not white knuckle the bars and push both knees towards the top tube.

I like to squeeze the top tube with both legs, seems to aid in stability. Also, as sketchy as the super tuck looks, I feel way more confident descending in that position and have never had any issue with speed wobbles at 40+mph.

that theory behind clamping the top tube with your legs is that it shifts the center of gravity, which disrupts the "harmonic convergence" that causes the high speed wobbles. (I think it actually raises it a little, by taking load off the pedals.)

I rode 808 and a disc.

Wonder if it is size weight related. 6’7 215 lbs.

I’ve had a lot of speed wobble issues in the past. To my mind it’s a geometry puzzle to which nobody really has the answer. All dependant on bike size, weight and height of rider, wheels, speed, road surface etc etc. All I know is it is pant shitingly scarey.
All I would suggest is testing your position on descents (or when you get it the most), by trying to get as low as possible. I also found wheel choice could cause it, so much so, I switched to a basic Shimano RS 010 (I think) front wheel for my last Lanzarote race. A zillion spokes, built like a tank and didn’t miss a beat. It looked pretty crap on my bike, with a deep section rear, but I don’t recall thinking during the race, “I wished my bike looked cooler”. I was just descending as fast as I could with complete confidence.

I've only had it happen twice, but it was the same location: the Keene decent on the IMLP course (luckily it was a training ride both times).

First time - tri bike, race wheels, in aero when it started, doing just over 50 mph. It wasn't a "wobble". My wheel was going 12-16" from one side of center to the other. I remembered thinking at least they just paved the road, so there shouldn't be too much gravel in my road rash when I go over the handle bars. I was able to slide back from aero to the hoods and squeezed the handle bars with all my might to bring them back to center. (I know this wasn't the smartest move, but it's the only thing that came to mind at the time)

Second time - road bike, training wheels, on the hoods, 2 years later, and I was a bit heavier. I had lost my confidence to descend fast after the first wheel wobble, so I was only doing 40mph. But it was the exact same location on the course. This time I was thinking, ok, you survived it last time, this is no big deal, you know exactly what to do. Yes, I was scared, but nowhere near as bad as the first time.

Unfortunately I rarely allow myself to get into the upper 30's on any descent anymore for fear of it happening again.

*Usually*, speed wobble comes from something being loose somewhere. It could be a bad quick release, play in the front hub, play in the fork, headset, bars, etc. I'd bet that's the case with yours too. One test is to stand in front of your bike, put the front wheel between your legs and then try to turn the bars. If there's any give/play anywhere, that's likely the cause. But, just generally try to move stuff around and see what you find.