Fill series with click and drive goes berserk

When I try to fill a series by click and dragging the bottom right hand corner of a cell (cursor changes to big plus sign), the cursor immediately jumps to a random row way further up the table, then as I drag down it jumps to random rows all over the table. I made a video of it but I don’t know how to upload a video so I’ve uploaded it to dropbox. Here’s the link.

Any ideas?
Thanks.

Test in safe mode, click Help > Restart in safe mode > Continue in Safe mode and test dragging there. If OK, then it could be an issue with your user profile