This sounds like a typical case of death wobble. I don't know how old your vehicle is, but this usually occurs when steering components, ball joints, and/or wheel bearings are getting worn out. With all the extra play in the front end, it starts vibrating/oscillating on its own, without driver input. When you drop down in speed, the oscillation quiets down, then you're able to get it under control and it stops happening, at which time you can speed up again and everything will be fine. It's really a strange thing to have happen. Some vehicles suffer from this far worse than others. To have death wobble, you usually need more than one part mentioned to be bad, as the slop in two or more of the items is what can cause the situation to occur in the first place.
To fix it, you need to figure out what part of the system is worn out. Start at your steering linkages and see if there is any deflection at all in any of the tie rod ends and ball joints. These parts should move freely in rotation, but should have absolutely zero deflection in any other direction.
- For the tie rod ends, you should be able to see the two parts move independently of each other when the the steering wheel is turned left to right.
- For ball joints, you have to see if there is any movement between the steering knuckle and the a-arms (assuming there's a-arms). You have to suspend the suspension them force the wheels to move in directions other than around. Again, you are looking for any movement other than rotational movement.
- For wheel bearings, lift the tire off the ground, then place your hands at the 12 and 6 o'clock positions and see if there is any movement. Anything other than the wheel going around is not good and could mean the wheel bearing needs replaced.