I just took delivery yesterday

I just took delivery yesterday on a 2007 BMW X3 Sterptronic. Car has 39,000 miles and is BMW Certified. Dealer loaned it to me for several hours of test drive. Car seems to have significant stumbling, surging and flat spots at tip-in when I'm just pulling away from a stop. Curiously, this event tends to be minimal to un-noticeable when statring from and driving down a mild descending grade. On my final trip back to the dealer, road conditions were primarliy downhill to flat and I think I erroneously concluded that I had qiuickly mastered the subtle and correct method of stating from launch. I bought the car. Shortly later, when driving up a fairly steep hill approachibg our house the engine and transuition exhibited very unsettling rpm, gear switching and jerking behavior. I've got 48 hrs to take this car back and return it. Very nice car purchased for good price, my instincts say run, don't walk away from this car. What do you think?