How is the stitch quality? If it is a balanced stitch, the tension is probably okay but it never hurts to clean it.
I read somewhere that the old girls were designed to work perfectly with the heavier weight of the old wooden spools and the new plastic spools don't always work well.
First, are you using a spool pin felt on it? That will help it feed evenly and not spin. This is what I'm talking about
:
http://www.shop.sew-classic.com/Spoo...of-2-8879T.htm
I've seen one person claim that taping a quarter to the top of the thread spool solves the problem. I probably wouldn't want to tape something and introduce something sticky, but you could probably try a zinc washer from a hardware store for the same purpose. Zinc wouldn't rust and the washer would have a hole for the spool pin. (I've used those zinc washers for curtain weights and they're way cheaper than the weights sold in the fabric store, too.)
Another thing that could be happening is that sometimes the thread feeding off the spool will catch on the notch on the end of the spool which secures the thread when it's new. Try turning the spool over and see if that helps--often, just that will solve the issue.
Good luck.
John