It's the support under the edge of the screen. The screen is taller. Lots of wind pressure. Just needs a ittle help at the
bottom of it.
If you're talking about the faired version, then the solution is to put rubber or thick foam between the shield and the fairing at the point where the shield starts to go above the bikes fairing. This acts as a brace. I had the same problem until I did this, even without the MRA screen. With the MRA screen it was awful. Bracing the CB shield like I said solved it completely.
In case I wasn't clear...
If you're sitting on the bike looking down the CB shield and you see a gap between the shield and the bikes fairing, that gap is what needs to be filled with something firm enough to support the shield in heavy winds. I've gone 95 mph with the MRA and I get zero flex.