fluctuating RPM

My 2000 windstar 3.8 has a slight fluctuating RPM at idle. It seems to occur every time when I stop at traffic lights but would stop after 5-10 seconds. I can however feel the slight vibration at idle all the time. There is no CEL. The car runs fine but it's just annoying at idle conditions and I would like to fix this if possible.

The car had new plugs and wires 5 months ago, new air and fuel filters. IAC motor looks clean, isolator bolts are new and EGR ports are clean. I just had the fuel injectors serviced and new coolant temp sensor. I measured the fuel pressue and it's 39psi with key on engine off and about 30 with engine on so it seems normal.

How do I go about to diagnose this problem? or is this the nature of the 3.8 engine? I got the car used about 1 year ago.

I appreciate any idea.

I disconnected the vacuum lines on the back of the plenum at the EGR vacuum solenoid and tested it with a pump. It can hold vacuum ok. After connected it back the car idles a little bit better but still slightly rough so vacuum leak was probably the culprit to my rough idle issue.

My scantool indicates short term fuel trim for both banks around 0-2% but long term fuel trim going around 10%. Does this mean I still have vaccuum leaks elsewhere since the computer is trying to compensate by going rich in fuel?