2009 Vauxhall Corsa power loss

Hi I bought my car in July, the day after I picked it up, I was going uphill on a dual carriageway the car engine management light came on the car seemed to hiss an lost power, I was doing about 65 in 4th gear about to change to 5th at 3000 revs. I took it back and the mechanic said after about an hour or so that diagnostics didn't show a fault, he blew some choked up pipe out in the middle, but couldn't reach the whole of it , he said after test driving the same hill, it was working fine, but if it happens again to book it in for 2 hours (he said it will be done under warranty )so he could take the 'pipe' out and de-choke it ( sorry not sure what pipe it was) the car was working fine, been on two long journeys, so had a good run until Christmas, then it started again getting a good run about 65 in 4th, I went to overtake a slow lorry near the top of the hill, the engine light came on with a sort of hiss and it lost power, now it seems to be doing it all the time, the garage put it on diagnostic machine and can't find a fault, I don't know what to do next as only had a 3 month warranty, they are happy to check again, they said if the engine has been turned on/off a couple of times, the fault clears so wont show the fault, is this correct? I said if that's the case may be the mechanic should test drive it to hear and feel the fault is that fair? thank you in advance for any advice /help