That old laser of mine did the same wobbly line thing. Ended up just using a manual 4-foot level on the wall. Marked two spots, snapped a chalk line between them. Took maybe five extra minutes but it was actually straight. Sometimes the simple way is the one that works.
My cousin's so-called self-leveling laser tried to draw a line for a shelf last week. It looked like the diagram for a roller coaster. I guess "self-leveling" just means it picks its own favorite angle, and it's never straight. So much for that feature.