Bruce - The symptoms you describe are often the results of a long standing but undetected problem with the clutch not completely releasing from the pressure plate. It usually begins with an almost undetectable difference in the way the clutch functions, then is followed by the sudden onset of failure somewhere in the clutch mechanism that is the result of overstressing the system.
I had a similar thing happen with my '66 912 (Although it didn't have a hydralic clutch)...The main seal began leaking oil onto the flywheel and contaminated the clutch surface. At first it was just a little "grab" and I thought it needed adjustment, but before I got to it, the oil permeated the clutch plate which then "cemented" itself to the flywheel...The result was a bent throw-out fork.
If it isn't clearly the slave or master, I'd start at the flywheel and work my way out.