Similar Problem to me.
I had a similar problem with my 540i, i also have hit 80k. It was fine idiling, but dead sluggish tootling about and when you wanted to jump out anywhere it didnt want to go, then all of a sudden your heads thrown back as it decides that you put your foot down 5 mins ago.
After lots of googling and not much help from BMW, i found that i had a faulty MAFS. If you look from your air filter box then run along the air intake hose there is a round cylinder with a sensor screwed into it, its easy to spot. Its a Bosch sensor and when you buy the sensor it comes with the round bit too, so dont worry about buying the funny torque screw drives with the centre hole in them.
I replaced mine and it was like someone had bolted on more engine, not only did it solve the problem it was a damn sight faster too. The MAFS isnt cheap, i was quoted about 180quid, i managed to get it cheaper, i have a friend who works at a parts counter for a dealer and he got me a good discount.
Do you have the yellow light on your dash to the bottom right, mine was on with this problem, it went off about a day after i replaced the MAFS.
ANy questions about this give me a shout.
"In a world without Fences who needs Gates"