Sputtering Jetta

My 2002 Jetta Wagon acts up when it is cold and wet. After being parked overnight when raining or even foggy, the emissions light comes on when I start the car. The light on the dashboard blinks and the car sputters and hesitates. The car has very little power when this happens. Once driven for awhile and it has a chance to dry out it drives fine again. Sometimes the light just stays on for a few days and eventually turns off. I have taken it in for repairs twice now. Once to have all the spark plugs and wires replaced and another time to replace a faulty fuel injector (this was pinpointed using the diagnostics via car computer). After both repairs the car had the same original problem. I have also asked for opinions at several garages and at the VW Service Department. All have said it sounds like I need new spark plugs and wires. I don’t want to keep bringing it in for repairs if we can’t determine the main problem. If the car were older, I’d think it was a cracked distributor cap. Please help me figure it out. I’ve already spent over $800 that I don’t have to spend! Thanks.