Mustang 2003 V6 shake (Oscillate right left) on bumpy Road ?
Righto ^. Check tire pressure, otherwise, the way the car sticks to the ground i.e. tread design, you'll feel every crack/bump/blemish in the road because that's the way it was designed. It sounds like the bump is catching your tread just right and shifting it fast, then regrouping itself. Avoid the bump, no more problems.
Another thing... if you notice the steering to sway back and forth, or feel the car start to shudder under speed, that may be a seperated tire, and if that's the case, tell Firestone to shove it!