Bete,
Hello there, long time no hear from ...
.
First I'd go with the basics. Clean it thoroughly. The problems you're having sound like top tension problems to me.
Before taking the tension off and taking it apart for cleaning I'd get a thin strip of fabric, maybe a 1/4", fold it and soak it in alcohol then run it around and between all the disks to make sure they are clean and free of oil, lint and fuzz.
Clean the bobbin area thoroughly as well. Then rethread it and try again. Also try a new bobbin. Those Sew-Classic sells are wonderful. A worn, bent or one with flaking plating can catch and mess up your tensions too.
Also Miriam has a thread here about tensions that should have lots of things to check: {
http://www.quiltingboard.com/vintage...s-t170748.html }.
That's about all I can think of right now.
And .... Minerva says Hi.
Joe