Clutch Disc

Could someone with an opinion. The problem is that the clutch of my 2008 Ford Ranger is slippery, but not all the time as it does when the clutch disc has disappeared, but sometimes! I was in the garage twice, say they have changed the whole kit. Whatever the problem is back, at times, especially on the highway! Some mornings, I feel the clutch "takes" at the bottom, as new leads after a little he "takes" at the top, like an old and slipping when I accelerate. Please help!

I have the impression that the slave cylinder leaking causing the problem.I should he return to the garage and a 3rd time to give them another chance to repairing.If they do not fix it leads to a Ford repair.Once that is done and the remedy is taken by the owner of the first garage in the small claims court to pay back your money because of not knowing what the heck they are doing .. Good Luck