Engine's RPMs fluctuating!!!!????

[Skimo100]

My 2007 x3 is acting up (only 20K miles on it). When the SAV has been sitting for about 8hrs on hot weather (80-90 F), I turn the car on, no problem there. As soon as I drive about a block or so and come to a stop and I'm waiting for the red traffic light to change to green, the car starts to rock back and forth and I see the RPM gage going up and down from about 500 to 1000 rpms. The problem is intermittent. As the car's engine warms up to normal operating temperature it stops doing it. The problem goes away, also, as I drive away from what ever stop point I was at.

Last time the car was at the BMW dealer (at the 15K miles service) I mentioned the problem and they said they couldn't dupplicate it. I has been doing it every now and then. Anyone had this issue with their X3 before?