SMD

I have a 2004 525i. Almost since i bought the car I have experienced some kind of surge or drop in engine power. I can physically see the tach drop and "bounce" as well as feel it. It does not happen all the time but it usually happens when i need to have controlled acceleration in traffic or on an incline when following someone. It seems to be getting more prevelant as of late. I have taken it in several times for service in regards to this and I am told there is nothing wrong. They have no idea what i am talking about and could not replicate it. The SA even supposedly took it home overnight and said the car did not do this at all. I drive it every day and know I am not crazy and imagining this. I have even had passengers comment on it. Has anyone else experienced this or have any idea what it could be?? It is driving me nuts and really making me think about getting rid of this car.