missing i think

ok i have a 1991 bmw 318i and my problem is that after you run the car for a while and it warms up it will start to studer when you go to 3500 or higher rpm. it gets worse when its under a load going up hills ex. I dont know what it is i was thinking map sensor or o2 sensor but im not sure. does anyone know what it is?
thanks,
Jamie