John is right, I'd be lost without my mutli-meter. 

You'll normally experience a couple of volts of drop from the draw at the starter, but if you're dropping more than a couple of volts, then you have a bad connection somewhere. If you experience no drop, then there is something wrong inside the starter. Just take the multi-meter and set it to read DC, then place the red lead on the ground (if you still have it set up for positive ground) and the black lead on the stud that the battery cable hooks to on the starter. Put it in neutral, have someone hold the clutch down and crank it.

My first guess based on what you have told us is that the brushes are worn out in your starter. My Super 88 did the same thing last fall, but a quick test with the multi-meter told me where to look. One new set of brushes, and she's spinning like a top again.