I replaced an alternator, battery, fuel filter, and had a tune up all within a week. The work was done at two separate mechanics. Everything seemed fine until it turned really cold. Now if the car has been sitting for more than a couple of hours, it starts up but runs really rough, and will usually stall out on the first try. I have to pump the gas pedal like a crazy man to keep it running.

My questions is who should I take it back to; the place that did the tune up or the place that replaced the alternator and battery??

Neither. It was a sensor problem. $50 fixed.