lately i've been having issues with the car starting on a cold temp. it feels like it's mis-firing, idles at 500rpm or less but after a couple of seconds (i mean like 5-15secs)idle jump to 1k rpm or so and operate as normal. it started doing this for about a week or so and now is getting worse. about 2k miles back, i have changed out the plugs, cap, rotor, fuel filter, air filter and o2 sensor.The engine is a rebuilt D15b7 with about 10k miles on the engine itself. has anyone experienced this same symptom on a cold start? Throw me some ideas of what could be causing this sort of problem. Thanks.