Since I purchased my '93 Explorer in April it has run poorly. I started out by changing the spark plugs and air filter. It seemed to fix the problem for about a month or so. Now it is occasionally running poorly and has started to backfire when going up slight inclines in the road. When I put gas treatment in it or run Super (expensive) unleaded it seems to run better.

Any suggestions would be appreciated.

Green93

Try cleaning/ replacing your mass airflow sensor. There are many posts about this procedure which you can search for.