I had an intermittent problem with my 1988 K100LT starter for the longest time. Starter would not crank with a good battery. I assumed that this was a battery/alternator failure. Took it to dealer and described problem. Dealer could not duplicate problem so did not do anything. When time came to drive home, I had to jump start it because it would not crank. Sounded like somebody was holding the starter from turning with a big screwdriver.
I gave up on the dealer and decided to tackle the problem myself. Found the problem the minute I removed the starter. When I pulled the Core out of the housing, one of the 4 magnets came out with it. Seems that the magnet had just enough room to move so that it would occasionally jam the starter and keep it from rotating. Any jiggle to the bike would move the errant piece just enough that the starter would work again. I replaced the whole starter with a used one and have had NO problems since. The glue holding the magnet apparently failed and allowed it to "float" about. I believe that a similar problem was mentioned in MOA but I thought perhaps you would want to post this in your wonderful site as well.
p.s. I just took a 250 mile ride on a brand new Russell "Day-Long". Everything I have ever heard about them is true. I experienced absolutely no discomfort in the posterior region.