1993 Plymouth Voyager Excessive fuel use

On my '93 Grand Voyager I usely get about 360 miles to a tank of gas. For the last four weeks I have only been getting about 250 miles per tank, and it is getting worse. I have run fuel injection cleaner through three tanks of gas with no results. I had the fuel pump and filter changed about 3000 miles ago. I checked the fuel pressure regulator lines for fuel and found no fuel - which is good. A complete tune-up was done about 6000 miles ago. I have noticed also when it sets and idles it kind of revs up and then back down over and over again, and the exhuast smells rich and to high heaven. Any ideas what the problem might be?