Does she have a heated water pail that can be used when the temperatures drop? I am assuming that she typically only colics when the temperature drops, so the cause of the colic logically has to be related to the colder temperatures, unless she also colics when the temperature abruptly gets warmer. Some older horses can be sensitive to drinking very cold water. Heated water pails are not exactly cheap, and keeping cords protected can be a nuisance, but it might be worth a try.
Is she in a boarding situation or at home with you?