My 04bmw 325xi hesitates shifting out of low gears,but it's only occasionally.

What is your question?

I can restart then it will run fine. For example, I'll take off at a light and it will sputter and hesitate, then finally shift and accelerate. Once I'm going it's ok. It will keep doing this until I restart the car. It's usually after I have my car parked for a few days. Recently it's happening more often, then today it would start, but stalled when I gave it gas. We let it go for a few hours, came back, the car started right up and now drives fine. !!?? I've had it in the shop for this before, they changed a sensor, but it didn't help. The mechanics were as puzzled as I am. They couldn't get it to malfunction for a while, and nothing ever showed on the computer.