Singer 201 Sewing Machine Repair?

We purchased a vintage Singer 201. This cute little machine with the potted motor was Singer's darling at one time. Our machine motor runs just fine, when disingaged from the head. You can manually turn the wheel with difficulty. The machine will run efficiently if you turn it over on its side! As soon as my husband tips it back up, it starts to slow then stops like it's catching. Any ideas? I have taken it apart carefully and I can't find a thing wrong with it. Thanks.

By MAW