You mean the clutch is slipping?
If you want to test if it is the clutch then drive up a hill (preferably a fairly steep one) and whilst at about 30mph floor it in fifth gear (or top gear) and hold it there. If the revs redline and the car isn't accelerating, that's your clutch going. This will not damage the car, it will just shorten the life of the clutch a bit more and every time it slips. This is not necessarily anything to panic about. I ran a rover into the ground with a slipping clutch for months with no probs, except on really hot summers day when it got bad.
If you want to test if it is the clutch then drive up a hill (preferably a fairly steep one) and whilst at about 30mph floor it in fifth gear (or top gear) and hold it there. If the revs redline and the car isn't accelerating, that's your clutch going. This will not damage the car, it will just shorten the life of the clutch a bit more and every time it slips. This is not necessarily anything to panic about. I ran a rover into the ground with a slipping clutch for months with no probs, except on really hot summers day when it got bad.