PO121 uneven idle at start up
 

2001 VW Cabrio GLX with 93k. Auto. When the engine is cold and I start it up the idle will pulsate, unven about 1100 rpm. When the the secondary air pump turns off the idle instantly returns to normal about 850 rpm. The exhaust stinks during this time like rotten eggs. But when the idle is normal no stink,. Throws PO121 everytime. Throttle body is clean as a whistle. All hoses tight, many are new. Any ideas? thanks for your help.