NEED HELP, Perfomance Problems !!!!

Any advice would help. I have a 92 3.35i. Not always but oftentimes when the car is in idle its runs extremly rough and shakes. The automatic transmission will occasionally not upshift when in "Drive" I have to put it in "3rd" to get it to shift. The car also seems to lose power in acceleratation.
I have taken it to the BMW mechanics and they ran a computer diagnostic but could find no error codes. I am wondering if it is a problem in the fuel pump or injectors, maybe a sensor. Please help! Thanks!