I had a very similar problem on my 94 acclaim. All the gauges on the instrument cluster would start going up and down all over the place, it was related to turn signals, lights. If I smakcked the dashboard a few times it would fix itself, but only temporairly. It turned out to be a bad solder joint on the back of the cluster. I had to pull it out and resolder it all. Problem solved.
I would suspect electronics and not the engine itself first.
1985 190E 2.3L - a constant project.