KenmoreGal2

I am using my avatar, a 1958 Kenmore made by Gritzner-Kayser. My top stitches look fairly good. Here's a photo.

[ATTACH=CONFIG]507633[/ATTACH]

However my bottom stitches are a mess.
[ATTACH=CONFIG]507634[/ATTACH]

I am sewing fairly lightweight fabric. I am using a size 11 needle. This is a new one, just put in. It replaced a brand new Organ needle, same size. That one kept breaking thread. This one does not break thread. (I am using polyester thread.) I can't figure what to do because it doesn't look like a tension problem, at least not the ones I see pictured on the web. I started with a size 12 needle, I've used 5 needles getting to this one. Same problem with all of them. Could I have a bunch of bad needles? Could it be something else?

Thanks!!