Rough idle on cold start

[rmathias]

1998 Jeep Grand Cherokee

, 6 cylinder, 4.0L

Purchased September 2009.

After driving vehicle for about 3 months, first, noticed RPM surging around 65-70 mph, highway driving, almost like a hesitation where RPM would surge up and then come back down – no fluctuation in speed. Runs great at slower speeds. Check engine light has come on a few times.

Next symptom just started about 8 months later – starts up fine, but after about 2-3 minutes, idles rough. Will have to wait about 2-3 more minutes, and then can drive with no rough idle and then runs great. This happens when starting up after sitting overnight, and also when sitting all day in the parking lot at work. Still getting the RPM surging while highway driving.
Changed the throttle position sensor but still idles rough. A mechanic buddy of mine said it could be the fuel pump? Any ideas would be helpful.

[fredjacksonsan]

Plugs, cap and rotor -- how long since replaced?

Using any coolant?



The surge on the highway might be the torque converter going in and out of lockup...so:

Transmission fluid level ok? Has it ever been changed?