Is the engine surging at idle or when you are going down the road?
idle surge is usually called a lean surge and an indicator you have a massive vacuum leak. 4.7's had a plastic intake and I have seen more than one that was split.
surge under load could be any number of things, but a quality scan tool is important to diagnose the issue.
If you have access to a scanner with live data, when idling, does the scan tool RPM match the tach RPM?
what are your fuel trims like? negative numbers mean the vehicle is taking fuel away because it thinks its running rich. where positive numbers are adding fuel because it thinks it's too lean.
if your fuel trims are negative or positive in the double digits you have a problem.
The devil is in the details, because a misfire and a surge are two very different things. if the vehicle is surging it would be described at a gradual change of RPM with a fixed throttle input. a misfire would appear with no change in RPM but a light to violent stutter in the smoothness of the motor.
cruising at 80 doing 1800rpms would make sense if you had 33-35 inch tires with tall gears. I have 3.92 gears on 33's and I cruise 75-80mph around 2000rpm.
IAC would only have an effect on idle performance, once you move the throttle the IAC doesn't really change anything.
How did you check the coil packs to determine they were good?
Personally I would use an infrared temp gun and make sure all the exhaust pipes at the head are close to the same temp, then I would pull the spark plugs and check them for condition and make sure they all looked the same with no oil, carbon, or raw fuel on them.
If your computer tells the scanner that you are idling at 700rpm and the dash says 500, it's probably time for a new dash.
another reason for a good scan tool is to be able to view a graph for the upstream o2 sensors to make sure they are cycling properly and not hung or flat.
