BMW 1997 528i

I am concerned that there is a problem with my car. After I break to a near stop, I press the gas pedal and the car would increase the RPM's normally but it wont accelerate untill a stong shock is felt and then it will accelerate normally. It feels like the car is not in gear yet and it struggles to get into gear. This happens if I accelerate fast after braking, if I actually do it slowly it wont happen. It is a 2.5 v6 and it has automatic transmition but it does not seem to be a transision malfunction, because when I do not pres the breaks this does not happen. I checked the brake fluid and it is fine. The check engine light is on also, and I was told it was just an O2 sensor or an emmision malfunction, which I do not think has anything to do with the problem but I might be wrong. I just bought the car and it has nearly 200k miles on it. I wanted to ask a question here before I take it to the mechanic, because I like to fix it my self if possible. Oh, the brake light is on too, but the brakes function normally. If anyone has any ideas of what the problem is please let me know. I appreciate any help.
PS this usually happens when the car is still cold, but when the car warms up it happens very rarely or not at all.
Thanks