Pacing actor eventually moves off screen

twinpix

I have a pacing actor (per the Wiki instructions) that paces properly. However, little by little, it eventually moves completely off the top of the screen. Here are the actor's Attributes and Behaviors. Any suggestions?

Density=99, Friction=0, Restitution=1, Drag=0, Angular Drag=0

Change Velocity (direction=270, speed =100, relative to: Actor)

Timer (Every: .25 seconds, Run to completion)
-> Change Attribute (self.Motion.LinearVelocity.Y to -self.Motion.LinearVelocity.Y)

twinpix

PS...I've noticed that the longer the actor is on the screen, the more it inches up slowly.

Thanks in advance for any suggestions!