Yeah, going really slowly solves it (as in snails pace) so I was doing that for topstitching, but it's so infuriating when I know I could have it done by the time I've done one channel at that pace haha.
I may try a better brand of needles as I do use Korbond and it's only a little more expensive for a good brand. I'm going to try Jeans needles, too, as I have heard they are good for corsetry/ sewing coutil. I have the Anne Ladbury "fabric" book, somewhere, so in theory if I find that it will tell me what thread and needles to use for optimum results

Thanks so much for all the help, I think it really is a trial and error thing to see what helps (I've been using the teflon foot on my machine recently to see if that makes a difference)
I do have my machine serviced once every 6 months to make sure it is working well, so that shouldn't ever be the issue.