I've had my new shape 520 (04 registration) for several months now and generally it is an excellent car. However, It has had a problem from new which the local dealer (Sytners Leicester) seem unable to cure. I wonder if anyone else has experienced this. Basically the problem is as follows:

The car generally seems to lack power between 1-2000 rpm, but it is intermittent. The main issue though is that if I drive the car within say 4 hours of stopping (ie when the engine is partially warm), the first roundabout or T junction I get to the revs die as I'm pulling off. In other words as I accelerate, the revs will rise to say 3000, and then immediately drop to 1000, then rise to 3000 and back to 1000. Once this cycle has happened, the car will be fine for the rest of the journey. The cycle of the revs rising and dropping perhaps takes only a couple of seconds to go up and down 2 or 3 times before it clears but results in the car slowing considerably.

Sytners have changed the following (allegedly):
Airflow sensor, ECU, temp. sensor. They have also done a software update and the onboard diagnostics have apparently never shown a current or previously logged fault.

I'm at my wits end with this as the car is dangerous to drive. On several occassions I have pulled off from a roundabout when the problem occurs and the delay while the engine "pulses" means I'm invariably confronted with a 38 tonne truck bearing down on me.

Any advice/experience of this problem would be much appreciated as frankly Sytners seem unable to help. I have contacted BMW customer services who will be providing a "pool" car for my use until the issue is sorted, but as they have previously had the car for a total of 20 days so far I'm not holding out much hope!

My last car did exactly that, the only way I cured it was to trade it in for my BMW.

My last car was a 14 year old 3.0i 24 v Vx Senator, you on the otherhand have a shiny new Beamer so I wouldn't have thought you should have such a problem as I did as I suspect my problems were caused by simply ageing of parts and years of muck and water attacking them.

You don't say what type of transmission you have.  If she is an auto it might be a fault with the tranny's ECU telling the engine's ECU to retard the ignition cos it thinks for some reason that the car is about to change up a gear

Here is a list of what I did to try and cure that problem - None of which worked for me but may work for you but like I say these things were changed due to the fact that they probably had past their age and mileage limits of useful service.

Auto gear box serviced and nothing wrong found but I'm not so sure.

7 New HT leads,

6 New sparkplugs, (all 6 old spark plugs were in good condition indicating to me that nothing is wrong with the engine)

1 new lambda sensor,
1 new crank angle sensor,
1 new air filter,
Dizzy cap contacts cleaned,
Rotor arm contacts cleaned,
Throttle body cleaned out,
Idle control valve cleaned out,
All the inlet air hoses de-greased and cleaned and re-sealed with new jubilee clips,
Dual ram hoses de-greased, cleaned and resealed,
Throttle pot checked and it produces a clean sweep on an analogue meter between each combination of the three terminals.

Temp sensor was checked by unplugging the cable from the back of the unit and driving the car - fault still appeared so that wasn't at fault.

Still juddered after the above so took her to Vauxhall dealer and they could only comment on the following,

Corroded LT terminals on HT coil,  

Knackered HT coil

Leaking inlet manifold gasket

Leaking injector seals,

Sticking throttle valves

Didn't bother investigating further, cut my losses and traded her in for my nice newer E39! 

Surely as a brand new BMW customer you should be able to give them the keys for the car and demand that you get a new one if they (your dealer) can't fix it?

Andrew