Engine revs sharply increase when clutch is pushed in during gear change and takes ages to return.

Hi i'm new to this forum. I've got a problem with my 1998 ford fiesta 1.3. When driving along in the car and you go to change into gear. So the clutch is fully down and my foot is totally off the throttle.So the engine is not under any sort of load. The car begins to rev up quite sharply and takes ages to return to normal tickover. I'm having to put the car into 5th or 4th to get the revs to decrease. As you can imagine this is a total menace when trying to drive around town or coming up to a junction. Its like it is drawing excess air in somewhere during that particular moment. I have checked that all of the air intake pipes are tight and nothing is looose, i have also checked the throttle body and condtion of the cable again this is all fine and isn't like it is sticking anywhere. I have tried various different things to try and cure it to no prevail. the only thing i have found which makes any sort of improvement.

Any ideas would be much appreciated.
Thanks.