Ok, I have had this problem a few times. The first time was the fuel pressure reg, and there is no code to tell you this... you have to figure it out on your own. The code said O2 sensor reading too rich. On my father's advice (licensed ford mechanic) with the mileage I had, we changed the O2 sensor. Shortly afterwards, I found out this was not the problem. I changed the regulator after reading about it on this forum... the symptoms were exact to what I had going on. Best way to test this is to put a gauge on the fuel rail, take it for a ride, and tuck it out of the way. When the CEL comes on, pull over and look at the gauge (even better if you have a long enough hose to run it outside and clip it to your wiper). Your fuel pressure should be 35-40PSI... mine spiked up to 90PSI when the CEL came on.
The other cause I had was a cracked plug wire. On damp days, engine cold, the thing would have a heck of a miss. When I took it for a drive, 5 minutes (getting up to temp under the hood) made the problem go away. The next damp day after I realized this pattern I popped the hood and started it up. Sure enough, a bad miss. Looking under the hood I could see some awesome fireworks coming from this plug wire. I changed it, it went away.
Back to the fuel pressure regulator... One characteristic that is common with this problem is that it usually does not rear its ugly head (CEL) until after about 15 minutes of driving. Once everything is warm, it seems to act up. You described the opposite. It could still be a possibility, but not likely.
Check fuel pressure at all RPM ranges, check injectors, check plug wires, heck even check plugs and wire boots... look for carbon tracking... you may even have a bad plug. Might even want to throw a vacuum gauge on it if you have access to one... you could have a stopped up catalytic, in which case you would have poor or no vacuum under any kind of load. Let me know how you make out.
