My 83 Rx7 runs fine above idle but stumbles and cuts off below 1000 rpm. At first it idles ok for a few seconds then stumbles for a few seconds, then idles ok for a few seconds, then stumbles again for a few seconds. Finally it cuts off. It acts like there's a vacuum leak that comes and goes periodically, maybe like a solenoid valve cycling on and off periodically, letting air into the intake. Is there some pollution control valve or maybe something like an EGR valve that could be cycling on and off, letting air in? I think I actually hear something like a switch clicking on and off near the carburetor. I've adjusted the idle screw to raise the idle above 1000 rpm to keep the engine from cutting off.

Any ideas?

thanks!