I have one of these wonderful machines too and sometimes I run it without thread and open the top and watch it to see how things are running I found a cracked plastic cam this way and replaced it. I think it could be like the other poster said oiled. I found that I needed to make sure there wasn't any lint when oiling too. I forgot some and the oil made it into a sticky mess. I hope you figure it out because that model was the best!
Lynda