I would try changing the signal lite flasher first, usually when they go the lights don't blink they just stay on continuous though. Or it could be the signal lite switch in the column. But my guess is the flasher.
If those things dont take care of it start checking your grounds. I had a simialr problem in my olds and it turned out to be a bad ground at one of the lights. Not sure if it's the same in these years but my 86 olds had a ground for every light fixture. I would thinks it's similar but never looked into it.
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.