Your problem was exactly like the one I had on my ’89 Honda Prelude Si. I could open the throttle when the engine was cold and the engine would accelerate normally. While driving, power and acceleration were normal while the engine was cold. But once the engine reached operating temp I had to feather the accelerator pedal at stop lights to keep it going and it would be hard to start if I failed. Power good if I could get going again and keep the RPM up.
Eric says to stick to the basics so I started checking plug wires, coil, and plugs. Lo and behold, I replaced the Denso plugs with NGK plugs and that fixed the problem. One of the old plug’s electrode and lower portion was brown. The other three showed evidence of carbon fouling, a symptom of rich running. I installed the old set 37,000 miles and 7-1/2 years ago.
I still can’t figure out why the stumbling problem would appear at low RPM/idle but only when the engine warmed up. I replaced the EACV with one from a junk yard 39 months ago but did not recheck now. The Fast Idle Vale is good. Vacuum at idle when cold and valve closed (no vacuum) when engine warm.
Nor can I understand why the stalling problem arose after just letting the car sit all last winter. It ran fine until the winter came but in the spring it wouldn’t start until I bled the cooling system. But even when I could start it again there was this odd problem of stumbling and stalling at operating temp.
However, there’s no denying that now after plug replace acceleration is smooth whether engine is cold or warm. No more dying at stop lights.