2005 Hyundai Elantra

For the past few mnonths, my clutch appears to be slipping, mainly in 3rd gear and up. It happens when I am shifting and also if I accelerate in a high gear. This will happen and then be kay for a couple weeks and start again. It is not a continuous process or getting worse. The car dealer tells me theat a clutch lining is not under the warantee, but another garage is teeling me it does not sound like a worn clutch lining. What could be other possibilities and what advise could you offer?