[tomw]

Mechanical problems, such as a burned valve, do not come and go. They are most noticeable at low rpms, and are in most cases hidden at higher rpms. Pretty sure there is a rpm limiter when poking at the throttle in neutral.
Throttle body would affect all cylinders.
Don't drive over 4,000 rpm and you will have less problems... "Doc, it hurts when I do this." "Doc: Quit doing that."
Rpms when driving 55-60mph should be in the 2k range, so a 4k miss should not be noticed. Given an intermittent at 55-60, then your rpms range should be lower. Have you checked fuel delivery? volume? pressure?
If no other symptoms, get the spark plugs back in good shape(replace), check the fuel pressure, air filter, MAF wires for clean, PCV for function, air cleaner,and see if the misfire goes away. Seems the plugs have been neglected. FIx that first.
tom

 

[tomw]

When you do pull the upper intake, I suggest disconnecting the throttle body from the intake(4 bolts) and moving it out of the way rather than draining the coolant and disconnecting the coolant from the throttle body. You have just as good access and don't have to drain/refill/spill the coolant. All the assemblies on the rear of the intake can be disconnected and 'left standing' by them self for the most part. IOW, they do not need special support nor to be removed completely.
I am not a fan of K&N filters as keeping them cleaned and properly oiled seems to be an 'art' that I do not claim to have. There is also a good possibility the oil can form a film on the MAF heated wires, causing mis-read of the actual airflow. The heated elements need to be perfectly clean and a little oil in the airflow is not a good recipe. Your choice.
Worn plugs can give stumble and hesitation and 'trailer hitching' when driving and would be most noted on acceleration up a slight hill where you do not get a shiftdown, but are working the engine harder than on flat ground, cruising. The plug breaks down(phrase) and misfires as the spark cannot cross the gap when the cylinder pressure is highest(under load, working hard...). Replacing worn plugs restores the spark ability to jump the gap successfully.
tom

 

[tomw]

I have done the R&R twice, using the same gasket over again. Once for plugs and the second time for cam cover gaskets & plugs. I was just careful removing, and fitting it back to the manifold. I kind of 'feel' how the throttle body sidles up the the intake and move it around until it feels as if it is in its spot, then run in a couple bolts finger tight to hold it in place. So far so good. If the O-ring style gasket has taken a set, and has no spring left to it, hardened, replacement is not a lot of dough. You can also wipe RTV onto the O-ring, no blobs, no daubs, just a 'finger wipe' to 'wet' the surface of the O-ring gasket. The RTV will set after while and more or less adhere to the O-ring and fill any tiny gaps. Some use white lithium on O-ring gaskets for the same purpose. There is zero RTV left to get squished out and be visible anywhere if you keep the application to a minimum. It really is just a wipe. But not absolutely necessary, and I have not used it on this application, nor the cam covers.
If/when you do the job, I would suggest replacing the PCV valve as it is difficult to get to, and likely has not been replaced. I have read that a failred PCV can pull a lot of oil out of the crankcase in short while, at least some report that. OTOH, it could be they thought they checked the oil regularly, and actually did not. I cannot say. I did replace mine. With Motorcraft, theFoMoCo OEM.
tom

 

[tomw]

I don't know about gas supply in your locale, but historically I do not replace fuel filters on any sort of regular basis. If you have fuel supply problems, the symptoms can vary based upon the problem. Climbing a long hill would place the most demand on a pump, driver and filter to produce enough flow over time to supply the engine. Accelerating as in merging onto a freeway would demand supply right now, but the demand would slack in a short while to cruise speed demand. A failing electronic driver would vary the pressure and delivered volume depending upon electron flow... it could change as quickly as throwing a switch, from normal do decreased flow/pressure.
In a closed fluid system, pressure depends upon restriction. If the fuel can flow freely, it will not develop any pressure. Once flow is restricted, pressure will build quickly, and drop just as quickly if the restriction is removed.
Checking the volume the pump can deliver is in some respects a better test than pressure alone. A pump can build pressure, but fail to deliver enough for an engine to run properly.
tom

 

[tomw]

Well, look at it this way. You are not lying in a mud puddle. It is not snowing or raining on you. The floor surface looks dry and almost level, and you can close the doors if the wind comes up.
I have heard that some people use an impact and run spark plugs out of the cylinder head threads as fast as they can. Also they get the engine warned up first. I have read that timesert may be a better thread repair than heli-coil. I have done a drain plug in an aluminum case, and used a heli-coil wannabe. It was not very successful as the plug screwed in at an angle when done and did not want to seal. The pan 'boss' was very think, which I did not know, and apparently I got it drilled poorly. The original had been kind of butchered when I bought the bike, so that likely got me off to an angled start. The metal was likely too thin for a heli-coil to work properly and I should have drilled & tapped from the get go.
It makes sense that an impact tool would tend to break the threads free of each other, but it just seems wrong to use one. I will have to think if it is a good way to go for me should I have sticky plug threads.
I should mention about RTV... There are brands that are "O2 sensor safe" that apparently will not kill the O2 sensors, so be aware that generic RTV can damage the sensors. I think a byproduct coats the sensor so it no longer has 'touch access' to the gases flowing past.
tom