Does anybody know what is wrong with my Fiat Stilo 1.9JTD Multiwagon?

My fiat Stilo 1.9JTD Multiwagon is playing up. I can start it, but cannot get any power through the engine. It then clears itself and goes for a little while, but then just loses power again. My mechanic has had it on a diagnostic but not found anything. He now thinks it could be the exhaust, he has disconnected it from the front cat and driven it over 5 miles with no problems. Can anybody shed any light on what they think it might be before i shell out £300 on a new exhaust. Cheers