Avensis Verso 04 loosing power

[KiwiDavid]

Please help! 

I have a 04 Avensis verso which intermittently loses power and slows down after driving it on a motorway for around 20 to 30 minutes and at a speed of around 60-70 miles an hour. When this happens, pressing on the accelerator makes no difference whatsoever, that is, the engine does not rev up and the car gradually slows down and then all of a sudden it starts accelerating again. 

2 local garages have run diagnostic tests which have failed to detect any problems.  I have read that it could be an issue with the scv (suction valves) and mentioned this to the garage. They said these cannot be replaced independently, the whole unit would need changing and that would be costly. 

I would be truly grateful if anyone could advise us on whether this is correct, or if they could recommend a garage that could change the valves. We are in North West London. Thank you. 

[FROSTYBALLS]

Petrol or diesel?

[KiwiDavid]

Sorry forgot to say diesel, d4d.