Rough idle on a half warmed engine

My engine has rough idle when it gets about half warmed. Once in a while it suddenly stops at that temperature on a traffic light. I have been having this problem for about a year since I’ve got 75,000 miles on it. It’s usually not that bad when it’s warm outside and it gets much worse during cold season.

I guess what happens is that when the engine is cold it maintains high RPM while idling, but when it thinks that it’s already “warm enough” (which apparently happens when it gets to about 50% of the normal working temperature) it starts to reduce RPM and I guess that it either does it too early or reduces by too much… That would be just my “uneducated” guess…

What is the problem? How can I fix it?

Thanks!

 

year make and model along with engine size would be a nice start.

 

I don't think that it will help, but here are the specs:
Daewoo Lanos 2000, 1.6L 4 cylinder DOHC, 16 valves, manual transmission.

The gas milage is perfect and it doesn't burn any engine oil. Also there seams to be no drop in a horsepower as far as I can tell. I also don't see any warnings as "check engine" light or anything like that.

Could it be spark plug wires? Maybe Oxygen sensor?