If you have played with the pump settings (main fuel adjuster, idle settings etc.) you may have too little pressure on the idle governor spring. This will make the idle unstable, higher when cold, lower when hot and make the idle speed very sensitive to changes in load. It also makes the car very easy to stall and difficult to launch.
To fix it you need to warm the engine up completely, back out the idle screw as far as possible or until the engine speed drops below 800rpm, reduce the residual screw until the idle hits 800rpm, then bring the idle speed up to your desired speed using the idle speed screw. This adjustment insures that the sensitive idle spring (and not the main governor capsule) is taking care of the governing duties at idle speed.