Hi Griffork -
Just for clarification, are you using absolute setting in your Velocity Over Life? Because if you are not then entering in a Constant 100,0,0 into Velocity Over Life will remove all movement in the Y and Z Planes (i.e. The Velocity is multiplied by 100,0,0), so your actual movement should be (1000,0,0) to (-1000,0,0), which is exactly what I am getting in my test project.
Eric Ketchum