Why is my stitching all WRONG?! (help)

I know some of you are sewing machine whizzes, and I'm very much a novice, so I'm asking for your help. What would make my (normally very well behaved) sewing machine make this mess on the underside of my stitching?

It will do this all of a sudden, and as it's underneath I won't see it until it eventually jams up. Then I take everything out and put it back exactly the same, and it's usually fine again. I can't understand it!

Is it my tension? Wrong bobbin (it's a drop in bobbin by the way)?

Any thoughts would be much appreciated!