My BMW x5 3.0i does not rev above 2000rpm and fails to drive uphill. The car is now a nightmare to drive please help urgently

SOURCE: 2000 ford focus stall/loss of power while driving

i ran into this with my focus. It turned out to be a loose connection on the crank sensor. It started right after i changed the serpentine.

SOURCE: the rev is going up & down on idle & when driving

have you checked your air inlet manifold for any leakages by spraying WD40 around each portall bolted to the cylinderhead gasket,check for vacuum air pipes deterioration.make sure the engine is running first ,then spray WD40 to check that the engine speed should increase.it sounds like your engine is haveing problems breathing due to heavy hydro carbons increasing./and your CO2% is dropping.

SOURCE: won't start, engine wont rev above 2000rpm

Have the alternator tested. You are providing a low charge to the battery when you jump it and when taking it above 2K RPM requires more amperage resulting in the engine dying and the rough idle

SOURCE: asc light comes on and engine cuts out

When your ASC light is on this is to let you know your Automatic Stability Control system is switched off press the ASC button Till light is off /ASC is on this works with your rev limiter and should stabilized your idle. If the problem persists have your throttle control unit checked as thi can trigger the ASC light as well and may also activate the ABS light if not fixed

SOURCE: my 2002 hdi 406 ticks

Sounds ike bad valve stem seals to me.