most likely cause is base idle speed not set correctly.
unplug the idle valve, adjust the idle speed with the screw in the bottom of the throttle housing to 850-900rpm - when you plug the valve back in it shouldn't change the idle speed, if it does then the base idle is still set too low. The idle valve should only be active to maintain idle speed when there is load on the engine, eg. fans, lights, aircon etc.