What front wheel are you using? I've noticed the same thing (with 404 FC on front). I tried with a standard wheel up front and it's a little better but still not great. I come from a MTB racing background so I thought my handling skills were above average but now on long fast windy descents I am reduced to a mere little chicken shttng in the breeze. The 404FC is meant to be more stable, but I think it's the combo of 404 and large surface area of the shiv up front.

Maybe I just need to get used to it. But as it stands I'm gonna lose a lot of time on races due to fear of wiping out on descents.

edit: PS looking at my race files, my speed instability issues seem to happen at around 40mph (i.e. my max speed is not higher than that)