Annoying Intermitant Fault

[craigdm]

My daughters R56 mini cooper, 90k miles. Last two times she has took it on the motorway it's started to misbehave after about 50 miles, especially when she comes off the motorway. Really bad misfire. First time the codes were misfire cylinder 3, so I changed all the plugs and the coil pack on that cylinder. Was working great, but today she's just done the same trip and a similar thing has happened. This time the EML light came on and it had reduced power. I cleared the codes and took it for a spin and all is good again, pulls cleanly, like a train. Codes were misfire cylinders 3 and 1, and 2B15. I'm guessing the misfire is a secondary cause, but I can't figure out the primary cause. I replaced one of the sensors about 6 months ago, and I'm sure that was the same code.

Any ideas??

[BBSparky]

Not running on Tesco fuel by chance are you? My mini was doing all kinds of hesitation and playing up. Swapped back to shell etc and has been fine ever since.

[craigdm]

Runs on whatever petrol is convenient ! but certainly not always supermarket fuel. Car actually ran smooth as a whistle all day yesterday, just pottering around town.

Got a feeling it's one of the sensors, just don't know which. The Stop/Start stopped working about 4 months ago, which may well be related as the battery is checking out just fine.

Do the garages have better diagnostics than the hand held unit I use?