3.1 miss

Hi I looking for a little help with my 1990

3.1 chevy cavalier. It idles ok with a slight miss but lately the power has gone away. It feels like it's running on 5 cylinders but sometimes it will clear up and run fine for a week or so. I have replaced the plugs,wires, coil pack, gas filter, air filter and cleaned the throttle body. Also about a year ago I replaced the crank sensor for a no start problem, and the map sensor. It has 195000 miles on it and i would like to get a few more. Thanks for any help.