With situations like this, any inexpensive "rule out" that also helps the car is a decent starting place. For example, a bottle of Techron injector cleaner will remove moisture and clean the injectors. (to a degree anyway) Porsche recommended it in a service bulletin for my 944 that the PO had kept, since then I have always put one in each spring. Mileage of the car would be helpful, if it's high and none of the spark components have been changed a symptom like you describe can manifest, (plugs, coils) a dirty or faulty MAF can affect that too. When was the last time she had an air filter? Those are some basic places to begin checking.