Cricket,
I have an idea. The bobbin case cushion spring may be set with too wide a gap. This will allow the bobbin case to move around more than it should and will mess up the thread path and tensions. I have had to adjust this on both our 500s.
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Check this picture out. There are four places that have adjustments and if they are out too much the bobbin case will jump it's track and your thread tension will not be right.
The specs are:
A: .012" to .016"
B: .012" to .016"
C: .015" to .023"
D: .025" to .045"
Check these measurements with a narrow gap gauge. C is a diagonal measurement between the little finger and the bobbin hook.
If they are out of spec, PM me your email and I'll send you images of the service manual where it has instructions on how to adjust things.
It's probably copy writed and if so the moderators will delete it if I post it here.
Joe