Idle control system malfunction.

Possible causes:
IAC valve mechanical malfunction
Fast idle thermo valve malfunction (A/T)
Throttle body clogged port, improper adjustment
Intake manifold , gasket leakage
Intake air hose loose, leakage
Vacuum hoses leakage (already stated)
ECT sensor
Throttle position sensor incorret output

If you don't have an automatic eliminate the Fast idle thermo valve as a possible cause.