Irishrose, as much as it pains me I will get some Singer needles ;)
Miriam, Gutermann thread perhaps? I have tried it, Coats N Clark, Connecting Threads, Aurifil and some cheap a$$ 2/$1 thread from Walmart that I'm trying to use up...lol. I knew the machine wasn't quite right when I bought it. I'm sure its probably something simple but those are the ones that drive a person the craziest..hahaha. I"m going to try moving the thread because it does jerk a lot. I've tried threadign it differently, don't remember where I read another way to do it, but I've tried that way. I really believe it is either in the tensioner or something in the bobbin case...just got to figure it out.
I play with them for a little bit then I have to put them away. For one thing, I never get any sewing done if I PLAY with the machines for very long :).
The 503A would probably right now run better, but its missing some parts and I'd rather get the "whole" 500A up and running and then think about what I want to do with the 503A.
I don't plan on giving up on the 500 even if I have to pay to ship it to someone to work on for me! :)