You can try this - use an invisible sprite with platform behavior (lets call it PlayerBox) and remove platform behavior from the player. When player is on this slope (use Overlapping at offset to check this), set Player position to lerp(Player.X, PlayerBox.x, dt*10), lerp(Player.y, PlayerBox.y, dt*10) , this should smooth out the jitter.
When player is not on the slope, simply set position to PlayerBox.x, PlayerBox.y