sticking revs

[dave.gsi]

was driving along today at about 35mph. coming up to a roundabout i slowed down and put the clutch in to change down but the revs stuck on about 3500rpm. i reved the engine a little higher and it went back to normal and was fine for the rest of the journey, but coming out of work before i was giving it some stick in second (bout 5500 rpm) and went to change into third but the revs stuck again. i had to rev it even higher and then let go before it would come back down to normal.

any ideas??