Clutch problems
 

The clutch in my truck has been slipping occasionally. I am sure that it is worn and time to change it, but the thing that is confusing me is that it will be fine for a long time, start to slip bad, and then be ok again. It seems to slip when I drive it for more than an hour or so, usually at highway speeds. I would think that if it was just getting worn then it would slip and keep slipping on a regular basis. Is this typical when a clutch is going out or do I have another problem, such as oil leaking on it or something else?