Ok, just a quick update. I'm more embarassed than I thought I would be!! I took that silver plate off and took out the bobbin casing... and there was probably a pound or two of hard-packed lint in there. OOPS!
I'm on my way to work, so it'll be tomorrow before I find out how this de-linting worked as far as my quilting/thread tension problem goes. But my machine MUST be happier!!!