spitting and sputtering all rpm ranges

i have a 93 mercury villager when accel it spits and spudders cked all sensors for proper restence ok cked fuel presure ok could it be that the dis. not advacing properly because it does it all the time how would i ck to see if computer is working properly advancing the dis.properly also it seems to clear up and run ok about 3500 rpm if you try to step on pedal it spitts an spudders then all of a suden it takes off or some times it will just dye off and start to slow down and wont re cover until you stop and go again please help