Having problems with my winter beater, hoping for as much input as possible. I've already got a few ideas from searches I've performed on related issues, but I wanted to get to professional opinions from as many people as possible.

QUICK SPECS:
1998 Honda Civic EX, auto, 2-door coupe
125k miles, stock motor, no mods.
Routine maintenance (oil changes, fluid levels, ..etc)
Everything is stock, just used for daily commute (mostly to & from college).
Driven normally, never beaten on... it's a stock automatic civic, I mean come'on!

THE PROBLEM:
Random misfiring on all 4 cylinders.
On cold start (winter or summer, doesn't matter), idles fine.
Once warmed to about 10-15%, begin minor misfires at idle.
When driving, random misfires for all 4 cylinders, but not more than 1 cylinder at a time.

**It would STOP misfiring after reaching around 2200-2400 RPM's.

WHAT I'VE NOTICED:
If left alone (when motor is cooled down), it idles and drives just fine.
Once it begins warming up, it starts random misfiring.
Going uphill, it will misfire continuously...
On flat roads, it will misfire if given too much gas... but if I let off on the gas, and lightly press the gas, it seems to stop misfiring until I either speed up or give it more gas to go uphill.

REPLACEMENTS, SO FAR (performed by a local certified Honda Specialist:
I started with the basic replacements (and cheapest parts) as listed:
After running diagnostics, it showed P0300-P0304 (random misfires from all 4 cylinders), and a bad O2 sensor in cat. converter.
(1) NGK Spark Plugs, installed and spaced correctly
(2) New OEM Spark Plug Wires
(3) New Fuel Injectors, and Fuel Line
(4) New O2 sensors for the header, cat. conv., and intake manifold (just in case)
(5) And of coarse, reset the ECU to clear the check engine light and error codes.

AFTERWARDS:
The car still seemed to misfire, but not nearly as much. Eventually, after about 100 miles later, it seemed to have gone away, but would still randomly misfire ONLY AT IDLE... so it still randomly misfired, but once or twice here and there at idle, never when driving (or at higher RPM's)
After about 3 months, it ran perfectly smooth. No misfires, no problems!
I stopped home one day after school to grab something, left the car running, and when I got back in and left, it suddenly came back again.
It seemed like it just came out of nowhere... only this time, it felt even worse. Ran diagnostics again, same codes!

MY GUESS:
I figured it's about time to try the distributor coil next, since it seems like the most logic issue after everything i've replaced thus far... but if that ends up being the problem, why would the car run fine after a simple replacement of plugs and wires?

I'd like as much feedback as possible. I'm not new to Civic's, I've been around them a long time (mainly B-series), so I'm not as familiar with D-series... Granted I could pick up a distributor coil for cheap, but I'd rather hear what everyone has to say before I keep spending money on fixing the problem if the coil isn't going to be the solution!

Thanks in advanced!

 

check your Cap and rotor first. might as well place them if they haven't been changed in forever anyways. google to check the coil before you replace it. and I would check the ignitor as well while the dizzy is open.

 

also check your valve clearance

 

Check this before going any further....

 

If it's a valve clearance issue buy some octane boost and run it through the system. If that helps your situation then you know it's valves.