My truck will out of nowhere acts like it partially disengages the clutch causing it to slip and then it derates the truck for a while. It happens 2-10 times a day regardless of load. I cannot establish any pattern or certain thing that causes it to happen. I've had it to the dealer and they don't know what to do. they ran clutch calibrations and it all checked out fine. Clutch is at 50% life. Any ideas where to start?