Low-rev engine bogging under acceleration, only when warm

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Hi, noob here seeking assistance! I recently purchased a 2.0L petrol E46 318 coupe, '02 with 55k miles. Immaculate machine apart from a faint coating of swirl marks due to the previous owners' addiction to car washes but the corning on rails sensation certainly makes a pleasant change from the "mattress on a skateboard" behaviour of my previous banger. Unfortunately though, two months in I seem to have landed a low rev hesitation problem.

The car hesitates under acceleration, in all gears, up to 2000 rpm, beyond which it is as smooth as it should be right up to the red line. Between 1000-2000 rpm the car bogs and kicks when put under heavy load. If you ask for light acceleration then it is not so bad but when you ask for some grunt it just stews.

Once in a �f*ck you car� moment of sheer frustration, I stuck it in 5th at about 1500 rpm and kept the accelerator pedal planted to the floor � after 5-10 secs of wallowing and kicking it actually cut out. One thing I noticed during this episode was rapid fluctuations of the MPG dial. With the pedal planted the needle was deflected fully as it normally is under full acceleration, however each time the engine dropped power the needle bounced back and forth to the 50-30 mpg sector. It seems that by command or otherwise the fuel pump was not supplying a steady stream of petrol. I�m assuming that it�s by command and some engine sensor is acting the maggot.

Also - the problem is definitely temperature specific - there is absolutely no problem when the car is cold and coming up to normal operating temperature. Once the temp gauge crosses the cold/normal threshold the problem kicks in and this division is quite noticeable. The worst it possibly behaves is when in slow moving city traffic when you would expect the engine to be at its warmest.

Fortunately, the car is under an AUC warranty so I�m booking it in for a check later this week. Unfortunately, the problem is slightly intermittent, however it is increasing in frequency. I will post the developments as they occur but I have a fear that as always happens the car will behave like a lady in the garage and the dealer won�t identify any problem or just say that it�s just an undocumented feature and you shouldn�t drive such a heavy car below 2000 rpm. 

As a heads up, i.e. so that I can say �did you check this?�, anybody have any clues as to what may be causing this?

BTW: for those who have used the AUC warranty, if the inspection shows up no problem am I liable for the labour costs or will the warranty cover everything?

James