I have read a few posts about slow/rough idle at running temp but I have not heard a consolidated opinion as to the cause. My '96 Civic (171k miles) does a similar thing. When it is cold it seems to run well but once at the normal running temp the idle tends to slow more than normal, like it wants to stall. Oddly, if I turn off all electrical devices, fans, radio, lights... the idle gets slower. Turning on the electric seems to trigger something to increase the idle speed to compensate for the increased demand on the electrical system. Mainly mine won't stall but it sure feels like it wants to. I can start it in freezing temperatures and let it run until it is warm and it will not stall. It just idles as smooth as ever. If I am driving it and I suddenly throttle down, like to stop, it acts like it can not compensate for the difference in throttle demand. I am wondering about a throttle sensor, or some other throttle related device. It is like the fuel system temporarily starves the intake a bit because it does not realize how much fuel is needed. Again, from cold to warm it appears to run normally but from driving to stopping it acts up.

TIA
KR

 

could be a fuel problem. i would check the timing and plugs first.

 

My civic was idling around 500 rpm, and I just replaced the grounds with a grounding kit. Now it idles normal at around 700 - 850 rpm.

 

I think your problem is similar to mine. Look farther down at the thread titled "Stalling". Appearently, this is a common problem with Hondas.

There is a "Main Relay" under the dash - above your left foot. This controls the fuel pump. When it gets hot, it stops working. My car runs
great always, idles fine when cold but stalls when hot.

The relay is about $50 or so from Honda.