Kingstowne Lawn -
Glad to hear you love the performance of your GrandStands. Per the instructions in your owners manual, we would suggest that you immediately try spraying the motion control linkage on the sides of the pumps with a dry lubricant product (NOT WD40, oil, grease or anything that will attract retain dust/dirt). Dry lubricants can be purchased at an automotive supply store. If this does not resolve the issue it should go into a dealer to be diagnosed. Hope this helps.
-The Toro Company