Hi, Sara. My experience with a "new" sewing machine was with a Husqvarna (sp?) dealer about ten years ago. Up till then my machines were older ones and I thought that if I were going to be doing so much quilting, I may as well invest in a new machine. I suppose I didn't want to invest ENOUGH. I informed my Husky dealer that I wanted a simple machine, a few ZZ stitches, but no computerized bells and whistles. She kept wanting to sell me a $2000 machine (sorry, I'm not going there). She eventually sold me one of their bottom line (bottom line being around $250 at that time) and assured me that it was a GREAT machine. It was loud and growly. It sewed no better than my old Singers. And I hated it. The dealer was willing to allow me my purchase price towards a trade-in. No, thank you. I eventually sold it to a friend for pennies on the dollar.
I'm including a photo of my vintage Husqvarna (then known as a Viking) in the basement that does wonderful ZZ stitches. I picked it up at a yard sale for $40. I knew by its age that it was built when machines were well-built. I'm not crazy about the bobbin mechanism but it sews beautifully. The tension on the decorative stitches is lovely.
I'm also including a photo of my Singer 403 which is my main ZZ machine. As Linda the Serial Quilter (that ominous name again!!! LOL) mentioned previously, the 403 is a GREAT machine. I've been sewing with this baby for about twelve years and have made tons of quilts with it.
Linda is correct in saying that there are lots of excellent vintage machines out there for inexpensive prices. With the manual, you can easily clean and oil these machines and keep them running forever. The newer machines are more difficult to access the innards, hence the need to take them to a dealer for regular maintenance.
Now...for your thread nest problem. What exactly is happening? Are you changing needles every now and then? A dull needle with a burr on the tip (from snagging the needle on the metal face plate for instance) can cause problems. So can putting a new needle in backwards.