I've a 2001 330i, and when I step deep into the gas pedal, the engine doesn't give that sudden power increase like it use to. When I down shift say from the 4th gear at 2500 rpm to 3rd 3300 rpm, in the past this would provide extra power and the car power sort of "jumps", now nothing happens, the acceleration is just very normal and flat.

I have changed the spark plug, engine oils, and there are no warning lights. I have unplugging the battery for a few months while in storage, so the computer settings would be factor default now.

Not sure what the problem is, any suggestions where I should look ? Thanks,