c200 coupe ignition problem

[siamese]

i have a 2003 c200 coupe,Tiptronic, mercedes-petrol..it is only running on 3 cylinders,when the engine is up to normal running temperature.The car runs perfect when driving,even when running hot,but runs on 3 cylinders when i pull up to a standstill,ie,under load..and when i turn the ignition off,and start the car again it runs fine,until i pull up again.The car has been on a diagnostics machine and pointed out that the black box (ECU)was at fault and not sending a signal to number 1 cylinder.The black box was replaced with a new one,along with everything else,from the box to number 1 cylinder.I picked the car up from the garage and drove approx 30 miles and the car started to run on 3 cylinders again,but only when i pull up to a standstill.The car is now back in the garage,a compression test was carried out,incase there is a sticky valve,but the test proved to be ok....Any suggestions on what the problem could be would be much appreciated