It could be the mechanical flasher, which is relatively cheap and generally easy to replace. On the '97 deville, it's under the dash just to the left of steering wheel column
There is also one on the right side, which is for the emergency flashers.
It could also be the actual turn signal switch and/or some crazy wiring issue [like a poor ground or power to the flasher].
As a quick, relatively easy and free test, try replacing the turn signal flasher with the emergency flasher and see how it works.