I doubt that anyone here will tell you to kick a 301 to the curb OR take it in. They're really simple machines, and I'm sure it's something simple causing the issues. Can you post a good clear shot of the bobbin case with the bobbin in and threaded through the tension spring? I want to see if there's anything we can spot. It really sounds like bottom tension, but I'm starting to wonder. There are also some suggestions you could go through on these two posts on my blog:
http://www.archaicarcane.com/fragile...-equal-stress/
http://www.archaicarcane.com/bobbin-...obbin-tension/