Probably not the switch or a fuse, or a relay. Most likely the linkage is binding, or the wiper motor itself is slowing down due to the cold and old age, or your battery voltage is low ( unlikely but worth a check)
Try soaking all the pivot points in the linkage with a good anti seize oil, if this doen't work, undo the motor at the linkage and see if it runs with no problems, if it does, i suspect its just dying slowly on you. You can try to replace the brushes in it, they may be shot, or take it to a motor rebuild shop and have them test it.
Good luck