Jup Liam, my solution does that. But now I understand it's not what you want. You just want an auto-scrolling background to suggest the effect of movement.
Can't you use a big active object? Because that works fine in most cases. Or is there something that prevents you from using it?
Ow by the way, you shouldn't use the "create" action, you just need to adjust the position of the scrolled active object so it can scroll once more. And if you're using the "create" action, make sure to destroy the other object you don't need anymore.
Let me describe it a bit more:
there is an active object which is your background and it scrolls off screen. Once it's off screen, you should move it back to it's original position and start scrolling one more.
Preferably your active object that represents the background should be twice the size of your horizontal resolution. Because once scrolling off on the left, something new needs to come in from the right.
You can also use a regular width background and put a copy of it on the right if scrolling to the left.
Interchange left and right if your scrolling in the opposite direction.
Keep in mind that you might occur some double scrolling because of the background itself and the entire game.