Hi guys. Ive got a problem

My car has no power and Im even used to it already. Engine reacts VERY slowly on pushed pedal, and this car is pretty slow itself.
But strange this happens randomly. Sometimes I start a car and it sounds differently(more roaring sound heard), RPMS are lower on idle than usual, and car goes like a sports car(so dont say something like: "Well 3.0 is not much for 2ton vechicle, and thats why its slow"). This thing lasts from the moment I fire up the car till I shut it down. Then car goes to the "slow" regime again. Feels like something somewhere sometimes in the car turnes on that is pretty important for the engine power. but 95% of the time, car starts and drives like ford escort 1992. BTW engine idle is smooth, and no problem with starting it.

Dont know what that could be.
VANOS?
O2 sensors? MAF? Air leak? Fuel filteR?

Any advice would be appreciated.

BMW 2001. 3.0i manual