[s-cars] engine at idle

[derocco_16v at comcast.net]

I have noticed that for the past few days the engine in my car has a little blip/skip when it idles. It is not there when cold, but once the motor worms up, the idle is not smooth. It drops just slightly and comes back to normal right after. No cut out when on boost, smooth ride in higher RPM. Where should I start troubleshooting? 
Thanks. 

Martin,