Get the fuel pressure and flow checked you may find if the fuel filter has never been replaced and it is on it's last legs. my one was bad when I changed it. Get the kit from mini don't use aftermarket one.
This is a how to guide http://www.northamericanmotoring.com...hange-mcs.html
Also drive the car driven with the diagnostic machine connected and check out the map sensor, O2 sensor and knock sensor checked and do a diagnostic snap shot as you drive it. Then you will be able to narrow it down
Also Check you vacum pipe to your fuel pressure regulator
You may have a bad sensor or injector that is not 100% correct but not throwing a code.
Get a leak down check carried out that will tell you if you have a bad valve or rings, it's quite quick and easy to do.
Hope this helps