Linda,
The dark bobbins may be the old machined bobbins. I have some that are the same way and if you look close at them they were turned out on a lathe. VERY EXCELLENT bobbins. I grab them when ever I see them.
Try better quality Class 66 metal bobbins in your 201, it was made for them. The plastic ones usually work as good as the metal ones but not always. Try scrubbing your bobbin case and check the bobbins it didn't like for defects.
The problem may be the bobbins, not the machine.
Diane,
The Dritz bobbins sold by JoAnn Fabrics are kind of a double level thing. The bobbins sold in the little plastic 10 unit box have been JUNK, while those sold on the blister cards have been better. At least that was our experiances a couple years ago when we tried them. I ended up junking about half of those we bought then and I haven't bought any more from JoAnn since.
Joe