What to do to fix cold misfire.

After sitting for the last month, the engine misfires at cold startup. Misfire evens when put under a load. No hydrocarbons detectible in the coolant. Misfire more severe as weather gets colder. Dealer mechanic pulled one or two plugs which appear to to be fine. Dealer mechanic says the rough cold idle is "normal" but I have driven the car since new and know better. Too, I have rented a number of these cars, including newer, and found that none of them start up with a rough idle. I wonder about fuel air mixture, which would point to the throttle body or the secondary air pump. Are there sensers there that could cause this condition? Other?