When you first start your engine there's no air flow signal coming from the MAS. The fuel mixture is determined by engine temp/O2 sensor inputs and some canned start programming in the ECU. Once the engine starts and air begins to flow through the MAS then it's signal can also be used to adjust the gas flow. Maybe there's some condensation developing in the MAS that's affecting the output when you first start the car and it takes a little extra air flow the dry of the heated sensor wire. Or maybe the wire is dirty although I believe it's supposed to go through some sort of "extra heat" stage to help burn off any potential deposits. Try reseating the MAS plug. Check your air filter too it may be getting dirty.