HELP! My 02 mini cooper going very poorly
My girlfriends 2002 mini cooper 1.6 was going perfect until it cut out on motorway, managed to get it back to mechanic but ran very badly. Mechanic has had it over 2 weeks and cant get it to go. He disconnected and blew out fuel lines with airline and said that they were very dirty, almost like black engine oil came out of them. Note there was never diesel put in tank accidentally or anything like that. He cleaned them and flushed out. Filled tank with fresh petrol and still no better. He then fitted new spark plugs and a replacement used coil pack but still nothing. He done a diagnostic check but reads no faults. Also he has put 2 injector cleaners through it. The car will start perfect each day he goes over to it and it will run perfect for sometimes 20mins and sometimes close to an hour then it will change dramatically as if only running on one cylinder and wont rev and when turned off will not start again? The mechanic is baffled to what is causing it. Any help or suggestions would be much appreciated..