I seem to be having a similar 'warm start' problem with my bone-stock, N/A M42. Car starts fine, idles fine when cold, but if I drive somewhere and stop, then restart the car within 10-15 minutes, it will almost never idle on it's own unless I keep the RPM's in the ~1500 range for 2-3 minutes. During this time, if I rev the engine up around 3K RPM, there is a definite miss and hesitation not unlike that caused by a rich mixture.
Looks like maybe I need to look into the head temp. sensor being the culprit. Thanks, JohnBoy!