93 Jeep Wrangler Clutch issue

I have a 93 Jeep Wrangler wit a 3 inch body lift and 97k miles. Occassionally my clutch seems to stop working, and I have no pressure in it and I can press it to the floor. Other times, it locks up and I can't push it all or it is very hard too, I can't get it into gear and it stalls out. Most often times this occurs after I put it in reverse, but it has also happened when I put it into forward gears while sitting in traffic. THe other day the clutch lost pressure, my gears locked up and yet the engine had a surge and the jeep jumped as if my gas pedal were stuck too. I have checked the fluid and it is never low and I have no leaks. I have taken the jeep in to a transmission specialist and been told that there is nothing visible wrong. About 8 years ago I had the master cylinder replaced and about 5 years ago I had all of my gears "rebuilt" (this may not be the correct term). Any suggestions of what the problem may be and how it can be fixed? Thanks!