engine surge

I have a 1994 c1500 with a 305 it has 200,000 miles and just last weekend it started to surge at idle when in drive. I changes the spark plugs and wires and it still surges when i am sitting at a stoplight it will drop from about 700 to under 600 and it feels like it is going to die. I don't know where to start. so any help would be greatly appriciated.
thanks
Mike

searching the forum for these issues might help.
Intake/Vacuum Leak - Idle Air Control(IAC) - EGR Valve - EGR Selenoid - Knock Sensor
Good Luck