Is it possible that you have used a spool of HAND Quilting thread? That kind of cotton thread is Never to be used in a machine due to the waxy finish.
If not, take your top thread and with the pressure foot UP, run the thread back and forth(like dental floss) in the tension area. You might have a little fuzz or broken thread in there and this will release it.

OK, I found my reference: it is from a Martelli magazine I got at the Quilt Festival. Also, here is the same information in video form from their website:

http://www.martellinotions.com/productvideos.asp

I read somewhere put your thread in the freezer over night: especially if 100% cotton. It gets dried out and frizzy needs the moisture just like we do. You said thread was very curly might be your problem if on your bobbin put that in freezer too. Who knows might find something good in there for supper.Good Luck.

I know that I had to tighten my bobbin tension on my 1500 recently because with so much sewing it had loosened.

Is your thread coming off the top of the spool - my SIL had a similar problem & she turned her spool around & was good to go.

I bought new thread (3 actually). I'm going to give a new one a try. Should I put the bobbin away with the old spool and re-thread a new bobbin? Or is the old one ok? I wasn't having any problems with the top just the bottom.

I checked this too, making sure the spool pin hadn't slipped down a bit. Nope, all clear there.

I would also suggest that you change needles. Especially if you have been using this one for a while. Mine was skipping stitches on a buttonhole the other day, changed the needle and the next buttonhole was a thing of beauty.

I would wind a new bobbin.

i had the same problem recently, i checked my bobbin case and found that the tension screw was ready to fall out. a quarter twist with the screw driver and things are back to normal