'99 Contour SVT stalls

The problem I am having is the car is stalling when it is cold, and running slightly rough when driving down the road. Once the car warms up, you can actually tell when the problem goes away, as the car abruptly no longer needs as much gas to maintain the same speed. After that, the car runs perfectly.

This problem has been very intermittent for a several years - originally it was a once-a-year kind of problem, and it would only happen when it was both cold (35-40 F) and raining. As the car has gotten older, it's become a crapshoot as to when it will happen, regardless of the temperature or whether it's raining or not. Now, as of yesterday, it has become a full-time problem, even when the engine is warm.

I have never gotten a check engine light from this. I have tried cleaning the throttle body and the Mass Airflow sensor, and I had the spark plugs and wires changed about 15K miles ago. Any ideas?