My 1995 Geo Metro wont start sometimes and revs up and down when it does start?

It gets plenty of spark and fuel but when it is running it also hesitates while it revs up and down, I have changed the dist cap, the IAC Valve, the Map Sensor and a few other things Ive looked for vacuum leaks and broken wires. I changed the PCV Valve and it ran ok for 4 days now its back to doing what it was doing. Has anyone had this problem? I am at my wits end!