My wife's 88 Voyager (2.5 liter) hasn't run really well for a couple of years now. Funky idle. Not smooth, etc. I recently installed a new timing belt. That helped some. But not until I installed a reman. power brake booster (Advance Auto Parts - $67 exchange) did the smooth idle return. The OEM booster had probably been leaking for quite some time but only recently did it make enough noise to get noticed and replaced. I'm happy and maybe you'll be too if this helps you.
Charlie