I have had fourcavi's. they all had this problem. I took it to the shop after I searched the grounds, the bulbs, the flashers, and the column switch. they were baffled by this as well. This is my fifth but first Z. Have not had the problem yet, but I'm waiting for it to happen.
Btw this alway seemed to happen in the really cold months. My car had no shelter then. Now I have this one in a heated garage.