1994 Buick Century Engine starts hard when cool outside!

My Buick Century in the last couple years has started to run pretty bad when its cold outside. All summer it runs like a charm but when the weather turns cold I start to have problems. In the winter months when I try to start the car first thing in the morning it starts hard and when it does start it misses for a while until the engine is heated up. It back fires and runs bad until the engine is warmed up. Once the engine is warmed up it has no problem running and runs normal. Everytime I try to take it to a shop the problem doesn't show up. I guess you can say its intermittent problem.