I've had two clutch cables snap on me during a ride over the last 40K miles and at least the last one was properly cleaned and lubed at the lever.
After comparing my low-mileage '94K75 clutch engagement action it was apparent that my clutch lever was sloppy at the pivot. I removed it today and replaced the nylon bushing which, compared to the replacement, was quite worn and wallowed out.
So, along with the cleaning and lubing of the cable barrel, check the movement of your lever for a worn bushing.