1-a bad DME temp sensor could also be the culprit, not letting the DME know the car is warmed up thereby creating a richer mixture and causing issues only once car is warmed up. Can be easily tested.
2-As mentioned, vacuum leaks can also cause surging/rough idle conditions. Harder to pinpoint but possible with compressed air.
3-ICV's commonly get gunked up over time, so this is worth checking, and cleaning if needed. (requires alot of work removing intake manifold, which is why its #3)
4-AFM worth checking if all else fails, but logically doesnt make sense why AFM would be a problem only when engine is warmed up.
PS: Our cars do not have a MAF (or MAP), which is a completely different type of air metering system than the AFM (barndoor) system our cars have. (Unless youve done a MAF or MAP conversion of course)
Troubleshooting - Engine