I use the following code to detect if a sprite should be going up or down:

If (pos.Y + 100) >= Sprite.BottomY Then
    Going_up = True
    pos.Y = Sprite.BottomY - 130
End If
If pos.Y <= Sprite.TopY Then
    Going_up = False
    pos.Y = Sprite.TopY - 1
    Vel.Y = 3
End If

Then my response code:

If Going_up Then
    Vel.Y -= CSng(gameTime.ElapsedGameTime.TotalMilliseconds / 40)
    pos.Y -= Vel.Y
Else
    Vel.Y += CSng(gameTime.ElapsedGameTime.TotalMilliseconds / 40)
    pos.Y += Vel.Y

End If

Sprite.velocity = Vel
Sprite.position = pos

But it's pretty terrible. It only works when the sprite starts at the top, and when I want to change the BottomY and TopY, it just starts glitching. What is a better to detect if the sprite should be going up or down?