Rough idling

My 1998 Honda Civic DX has 140446 miles and starts up and runs quite smoothly until the engine warms up. Then it begins to idle roughly after stopping say for a light or stop sign. Sometimes while stopped the engine idle smoothens out a bit. What could the problem be? Would appreciate any help.

If any of your routine maintenance (plugs wires filters) isn’t up to date then make it so. Any of those are possibilities for minor rough running conditions.

In addition you may have a sticky Idle Air Control (IAC) valve. Those can be cleaned.

Why any of that is warm car only I can’t say except that when the car is cold the computer keeps the “choke” on until it warms up (sort of).

Is the check engine light on?