01 Audi A6 1.8T Fault code P1128 Bank 1 Mixture Adaption Too

Hello
My car is running awful and doing approx 15 to the gallon. I have had 3 Garages look at it and all have drawn a blank.
Every time error code P1128 appears - Bank 1 mixture adaption too lean. I have had a MAF fitted, Lambda sensor fitted, a thermostat fitted, a temp sender unit fitted, the Throttle body cleaned, every pipe or hose checked for leaks and yet I still have this problem.
As well as the poor MPG the smell of burning fuel coming out the exhaust is awful. The EML always comes back on after approx 30 miles after being re-set then the power goes into limp mode and black smoke comes out the exhaust.
The car starts fine and it doesnt cut - out atall.
I am at my wits end with this problem not including the money I have spent on it. Please help if you can. Regards Steeley