1991 Geo Prizm Idle variations

Hello, We have a 1991 Geo Prizm that idles with a constant raise of rpms and a drop of rpms when the car has been warmed up. The car has about 120,000 or so miles. The timing belt was recently replaced, but did not affect the idle. The car never drops in rpm so much to kill the engine, but it does drop fairly low. The high idle parts are not above normal rpms, but do not stay there ever once the car has warmed up. Any help would be appreciated. I have a suspicion that the intake needs to be cleaned, but I don’t want to tear into things if they won’t fix the problem. Thanks, Charlie