Did you ever take apart and rebuild the upper tension? It is very easy to do - Tammi has probably the best info on that. Here is some more info:
http://www.sewusa.com/Sewing_Machine...Singer_301.htm
When I rebuild I pull the whole mess out and clean the inner pin - some times it can be a bit gooey and not want to function right. Dried up oil, lint, nicotine, some times fuel oil residue from the furnace... sticky hands, lunch Clean it all off and then adjust.