Engine rough/surges at idle

My 2003 Northstar SLS, 65K miles runs a bit rough at idle, sometimes a slight surge. I may be imagining things, but it seems to surge or hang while driving down the road at speed.
The car starts fine, hot or cold. The rough idle get worse after running at idle for more than 20 seconds or so.
No check engine light.
I have changed the spark plugs, air filter and cleaned the IAC, which only had a slight dry carbon build-up. Not much improvement.
I also checked around to see if any vacuum hoses were broken or disconnected but could not find any.
I would appreciate any suggestions.
After looking on auto parts websites, parts for this car aren't cheap!

bump

Codes stored? May need scanner to read ecm data to find cause.