I had a very similar problem. Did the trough test, had a poke around testing switches etc and must have inadvertently cleaned it up as I didn't have the problem again.
Then every now and then a ball would load and go back into the trough and next ball I would get two balls, as it would sit on top of the next ball.
Found that occasionally a ball would hit the corner of the spirit level and fall back down. Rounding off the corner of the level did the trick and haven't had it happen since.
