my veccy 2.2sri 53plate?

ok so im driving about 15 miles when suddenly the revs shoot up to 5k i depressed the clutch took foot off the accelerator and pulled over by this time i had lost a lot of power as well
restarted the car and it was fine then what it did it again had it plugged into the computer at dealers didnt find any faults
so now scratching my head whats wrong with it
cant do more than 10 miles before it happens
can it be the butterfly valve and if so how do i get to it to clean it or replace it