Since you said that the straight stitch looked OK, your machine is probably not far off. I worked in a shop for a while and these were our recommendations when someone was having these sorts of problems:

1 New Needle (make sure that it is in correctly)
2 Re-thread needle and take out bobbin and reload it
3 Remove Needle-plate and vac out fuzz - clean bobbin area as well
4 Use good quality thread (a lot of time I actually use embroidery thread for decorative stitching)
5 Use a light tear away stabilizer

and if all else fails - bring it in,

The needle size will depend on the fabric you are using. You are correct - many machines are finicky and you just have to find what makes them happy. Good Luck

This stabliizer stuff--what is that? I don't believe I've ever used it. This will be sewing ribbon onto faux suede which is on cotton.... so it's pretty thick.

If you are using Universal needles, try Top Stitch needles instead.

You should only use a needle for 8 hours of sewing...just think how many times that needle is in and out of the fabric in just 1 hour....the needle develops a burr and it will distort your stitches..

I change needles everytime I start a new quilt. It seems to work better and needles are less expensive than a new machine.

I, too, was going to suggest light weight stabilizer, but you're right about what you are working with being quite thick. So, I'm not sure that's your problem. I just know that my decorative stitches turn out much nicer when I use the stabilizer. JoAnn's sells it. It is called Tear Away Stabilizer. Comes in different weights, I think. You need the light weight. For small areas, I use a coffee filter as my stabilizer and it works just as good ...... and it's cheap.

That happened to me with my Bernina and I brought it in for servicing. There was some balance thing that was wrong, and it took the guy about 10 minutes to fix it! I would definitely bring it in for a service, or a repair of the one thing.

I definitely agree with the new needle and lightweight stabilizer advice you are getting in the replies. There are several types of stabilizer available. I find it a pain to tear away the stabilizer since it sometimes will distort what had been perfect stitches. Some stabilizers are meant to be left in the fabric so I often will use that type dependent on where the stitching is. There also is a type that disintegrates when ironed--very nice if the thread can take the heat. Especially nice with intricate stitches that would hold on to little bits of the tear away. Investigate your stabilizer options--lots available on the market.

I agree with all of this.

What size is the best? 11/75? I seem to have a bunch of 14/90--don't know why I bought those.

11/75 is good for piecing but I wouldn't use them for quilting or decorative stitches. I would use 80/12 or 90/14. I also use the Topstitch needles by Schmetz.