Alt light flashes, idle speed increases
 

While idling, my 1968 911L will suddenly flash the altenator light on for half a second and then immediately the idle speed increases 200-300 RPM for a few minutes and then settles back down to the previous RPM. What could possibly cause this?

when the alternator is charging, it creates a small load on the engine. it's not charging when the light is on, so the engine doesn't have that load and speeds up a bit. check your brushes.

Thanks for your input, John; what you say makes sense. What I'm confused about is,
why does the higher RPM persist for several minutes, after the "not charging" light had
only flashed on for a split second?
I'm going to pull out the alternator, follow your advice and check the brushes. I'll send
you some feedback on the problem as I progress, hopefully to its resolution!