idleing problems

When sitting at a stop light or sitting in gear somewhere, sometimes the RPMS will drop down to about 250 or lower, shake the car, then every now and then die for no reason, im thinking from what i read the Intake Manifold Gasket is bad but im not sure, its a 92 V-6 Rodeo. also coming off the Air intake box, underneath the filter, is an air line running to a Round container which i think is the Idle Air Control Valve, not sure though, but when connected i get a loud sucking noise that i can hear from about 30ft away, but when disconnected it idles better except for the stalls at stop lights, any suggestions?