How can I fix my Seat Tolledo 1.6i petrol?

I have from new a Seat Tolledo 1990 (x) 1.6i petrol, currently 28k miles on clock.
The problem I have is becoming more frequent and has happened a dozen times over the past months.

The problem is that after the car has reached running temp whilst driving, you change gear and the revs increase on there own to circa 4-5k and stay there till you,
1. Either switch the ign off (and after doing so it is not easy to start),
2. or sit there for approx 2 mins when all of a sudden the revs return to idle.

When this happens, you get no warning light on the dash. So does this mean that the fault would not show on a fault diagnostic machine?
Any ideas please.