I have a Ford Explorer 2000 Sport Coupe, 4.0-OHV, V6 - 4WD. I've notice that everything works fine on the expressway. But in stop and go driving after a while it starts accelerating sluggishly while moderately vibrating, also vibrates when slowing down or braking. Runs smooth as a top while sitting at a traffic light. The accelaration is not really that sluggish though. It's sort of like maybe one spark plug missing or somthing. These conditions are not always consistant.

Sometimes I pull off or slow down and everything works fine. But more often than not, it's doing what I describe above. What is consistant is that it always happens in stop and go traffic. Once I go over forty (or sit at a complete stand still) it's smooth operation.

Does anyone have any idea what this could be?

Thanks in advance.

PS-
I just changed the IAC (Idle Air Control) valve yesterday. But the described problem existed even before I did that and this did nothing, I could see, to remedy it. Changing this IAC valve did however stop my Explorer from loudly humming and whinning like a moose, periodically.