No drive above 3,000 rpm

Weird one.

Car pulls away with no noticeable slip up to 3,500 rpm, gets up to and cruises at 30-40mph no probs.

However, anything above 3,500 rpm in any gear I just get revs and no power. The revs come on fast as well as though the clutch is slipping.

Any ideas?

Cheers.
 
Just been out and gave it a proper run (45 mins through town, motorway and countryside) including a stretch of flat out through 1st, 2nd and 3rd and there was no sign of any probs this time. Will just have to keep an eye on it.