Its normal for the car to hold a gear longer when the vehicle is still cold, in order to warm up the cats. Our '99 E430 does the exact same thing in cooler weather. Try not to go about 3-3.5K RPM until the engine is warmed up and you'll be fine. If your car is actually losing power though (as opposed to simply not upshifting), then it could be another problem entirely. Any more info?