Start with a complete tune up, plugs, cap, rotor, wires, (use BOSCH ignition parts, USE ONLY W7DC or W7DTC plugs!!)<edit>W7DTC are NLA, I recommend NGK BP6ET instead </edit> air and fuel filters, replace your injector seals and idle screw o-ring, tighten the injector holders, replace the injector holders if needed. Replace all the old and frayed cloth covered vacuum hoses, silicone hose suggested. Check your Jetronic grounds. Test your oxygen sensor. Check your throttle switches and idle boost valves, check your CIS system and control pressures, check your distributor's centrifugal and vacuum advance mechanisms, set your timing. Set your mixture while monitoring the duty cycle of the frequency valve, adjust the base idle.

Also, to help put all this in the proper perspective, comsider the following quote from
http://volkswagen.msk.ru/vw_doc/eva2/FU01/ch3.html

"Do not try to correct engine trouble by making simple adjustments to the idle speed or the idle mixture (CO). Changing these settings without the proper measuring equipment will only be a hindrance when trying to locate the real fault."

A dwell meter or a DVOM capable of measuring duty cycle and a Bentley Repair manual are absolutely essential tools for any DIY CIS owner.

Once you've made sure all the basic stuff is working as designed, should you still find improper idle conditions, report back with your findings and we'll discuss it to a conclusion, more likely several conclusions.