I've put some thought into this, and I've come to the conclusion that it's a fault in the design of the DFP.
You see, while the wheel has force feedback, it has no vibration function. The only way for it to "vibrate" is to move the wheel via the force-feedback motors. When a car gets going that fast, it does shake a bit.. but it would be the kind of shaking that a simple rumble feature could simulate, like a standard controller.
What happens at high speed is that the wheel moves to simulate "vibration". Because of how fast you're going, that tiny little shift is enough to change the direction of the car slightly, which makes the force-feedback kick in yet again as you try to wrestle it back in the proper direction. This results in the frustrating side-to-side wobble.
Me, I only experience it when going 220mph+. Less than that, and I don't really have any problems. It also depends on what track you're on. At the Test Track, I can typically get going faster before the shakes set in. At LeMans, which is a real, bumpier road, the shakes set in earlier because the road surface is different. That's one of the things that led me to believe that the "shakes" are a result of the DFP's lack of a rumble feature.
In their next wheel, along with a clutch and H-shifter (and proper paddle-shifters.. those tiny buttons annoy me), they should add a rumble feature to the wheel to compliment the force-feedback.