1992 Chevy Blazer Surging

The engine is surging at an idle, and does not want to take fuel. I have changed the spark plugs, wires, distributer, cap and rotor, map sensor, throttle positioning sensor and fuel filter. The engine surges and wants to die at an idle. At a stop you have to feather the gas and hold the brake. Once you take off it will stay running. Sometimes it takes it a while to come up to speed, other times it is fine. If I unhook the vacuum to the map sensor it idles fine. I have also changed the intake gasket and throttle body gaskets. What else can it be?