[PhilBot]

Does it only happen when the joystick is fully forward (not backwards)?

If so, it sound like a signed to unsinged integer conversion problem.

Since the joystick actually puts out max negative when you push it forward, somewhere in the software is negating the number. Since there is a -128 in 8 bit signed math, but not a +128, this is sometimes converted incorrectly.

If that's what it is, you might also expect it to happen to the X axis when you move the joystick all the way to the left, but it may not since that value probably isn't reversed.

I don't use Robot C. What "type" is the joystick value (byte or integer)?

Phil.