Vauxhall Zafira struggling to keep power on exceleration

i have a 2002 vauxhall zafira.. about 18 months ago i had an engine management light come on, and the car seemed to lose it`s get up and go.. it seemed to struggle when excellerating, almost like if it did a massive backfire (or cough) it would feel much better.it was a sensor failure in the exhaust.. both sensors were replaced and problem solved. a few weeks ago i had another light come on.. took it to my local garage and the put it on a diagnostic`s machine and it seemed to be a fault with the co2 sensor.. i booked it in for repair, the light went out before repair but had part of the exhaust replaced as it was blowing, since then the car is struggling yet again, diagnostics are showing no problems with the exhaust, my local garage has had it for two days and tried everything and are baffled, the light that appeared briefly on the dash as i was driving the other week was a car with a spanner and also a car on a skiddy road???? the car completely lost all power so i pulled over, turned the engine off, restarted the car and the lights had gone out, but it is still struggling like it yet again needs to cough.. please help..