engine revving up & down

1995 Blazer 6 cylinder 2wd 190000. My enging is revving up and down very badly. You can hear it sucking in air as it revs back up. I have changed the plugs, wires, distributer cap & roter, fuel filter, air filter, egr sensor and throttle position sensor. I checked the o2 sensor near the exhaust line and it was clean. A friend of mine metioned that there may be another o2 sensor near the back of the moter...is this correct and how easy is it to do for a shade tree mechanic like my self? If that's not the problem, does anyone have any ideas? Thanks