I've just come back to this thread after a couple of days, and so many helpful replies! Thankyou all. I've also got in touch with Bob at Superior. The one thing I'm sure isn't the problem is lint in the tension discs, as both machines have just been serviced - I watched the Janome's tension discs being cleaned. And I don't think I can be threading them wrongly, as I don't have problems with any other threads. I'm going to try the 90/14 top stitch needle that Bob recommended, and I will sit down with the threading instructions just to be 100% sure I've got that right. Will report back!