general problems of the 2001 BMW Z3

I am driving as normal and then the car begins to have problems shifting and the transmission indicator light comes on. The owners manual tell you to pull over and shift the car into park and then apply the parking brake and turn the car off and then back on. It seems to temporarily correct the problem but it still occurs on days when the air temp. Is high. I have had the car on the diagnostic a number of times with no cure. As far as I know there are no recalls on the transmission or ecu. But this could be a major problem at high speeds on the highway or in city traffic if the car stalls or locks up BMW 2001 Z3 2. 5i (steptronic shifting).