Vauxhall Corsa lack of power

I bought this car for my patrner it was fine for first 5 months but now when you drive it either from hot or cold after you drive 800 yards and acellerate (like on a slip road) there is what sems a misfire on the dash and the car looses its power ! you can then pull straight over switch the car off and start it straight back up and hey presto its fine has power again for 800 yards then bang no power. The car has one previous owner and a full Vaux history 40,000 miles and no blue smoke or water in the oil ? I have changed the EGR valve (not a easy job) and also changed the MAP sensor on the manifold this also has not solved it ! I have had it on a diagnostic machine that says high boost is detected and the fault re-set but it just happens again 800 yards later. Can anyone help i am at a lost end the car is only 5 months out of warranty but my local Vauxhall dealer dose not want to know unless i pay the minumum payment of 75+vat for them to tell me what I already know about the high boost. I just want the problem fixed can anyone shed any light on what my next move could be ?