baskets4moo

My main machine is Janome 8900. I piece with Superior So Fine 50 wt and Bottom Line 60 wt in the bobbin and use a Superior Titanium 14 topstitch needle. I couldn't recommend Superior products higher. If you check out Superior's thread education you'll understand why cotton thread is an old wive's tale. http://www.superiorthreads.com/education/
Any time I've had a stitch "issue" it's because there is a build-up of lint in the bobbin area from using lesser quality threads (because I've needed a color) or my needle is dull.
Good luck and enjoy!

Tom W

You mention you are using a heavier thread. Depending on how heavy (40 wt or 30 wt) you might try a size 16 or even 18 needle and do use the new purple tip needles. My mom has an 8900 and that solved the issues for her, and she now has no trouble with any thread at all and stitch quality.

Dolphyngyrl

Maybe see if there is a yahoo or facebook group specific to your machine. I have heard this commonly with this model and those groups will probably give you better success. All of the people have different machines and it is better to get the question out to those who have you machine

MadQuilter

That is a myth. You can certainly use polyester thread. Does the machine skip with the same needle and different thread?

Did you clean between the toes (did you pull off the needle plate)?

jul12

I got my 8900 three years ago and had same problem. But have been using the purple tip needle since the first month - with all threads and no problems. FMQ with the needles is beautiful.
The Janome purple tip is made by Organ - I also use identical Organ HAx1SP Super 90, which has the same snake tip as the J.purples.