If there are times where the problem "just goes away". (and after checking to see if the HB bolt head is still there). I would first try to tighten up the bolt holding the crank sensor. If you get any movement at all from the bolt, I would reset the clearance on the old one and try it. If you just replace it, look and see if there are "scuff" marks on either side of the plastic channel. If there are, your sensor and HB sensor vanes were touching every now and then.
To adjust the clearance, u can use a match book cover BUT put it on both sides. (if one side, tightening with cause an ever so small movement of the sensor). This seems to guarantee that there will be some clearance on both sides. BUT before starting, maunally turn the crank to make sure nothing touches.
I had to join the SCCOA (thanks guys) to eventually get mine fixed. It went on for years (and finally bought my 2nd SC because it was leaving me on the curb) before I learned what it might be. From the posts, it usually self destructs when it goes taking the sensor and sometimes the harness with it. I guess I was one of the "lucky" ones where it just got intermittently worst. I guess touch, not touch, touch. Replacing the crank sensor (and adjusting it 3 times (finally on both sides)) fixed mine right up.
Hope this helps
--Fred