Recently purchased my first land rover - 2002 Range Rover HSE... got a pretty good deal on it I think, according to the seller it was his college-age daughters car... that being said it has had a few minor issues, just fixing little items as I go along, but seems to be a solid truck, and I love driving it (first 4X4 as well for me)
That said, I got a few misfire codes, so I took some advice from lurking around this site and installed replacement champion plugs, and magnecor 8mm wires. That's done, and I'm positive that all of the wires are seated on both the coil packs and the plugs themselves (checked it about a million times in hopes that it would fix this) The truck runs noticeably better, but still giving off codes P0307 P0300 and P0307 (twice for some reason)
Truck runs great at idle, and at higher RPM, but when I'm driving really conservatively and it kicks down to say 3rd gear at around 1200-1500RPM you can really feel the misfire, can feel it other times in gear and under load as well...
I have a pocket scanner Actron or something from autozone, and I'm not getting any O2 or MAF codes, pulled the #7 plug and it looks good, no oil or coolant, although it's only been about 50 miles.
Service Engine Soon light comes on and flashes. After I clear the codes it doesn't come on at idle, but when I put it in gear and drive it immediately comes on, and then at about 1500RPM, or maybe 15-20MPH it starts flashing.

Other notes/clues: I got the "gearbox overheat - refer manual" a couple of times, but it went right off. Also, the drivers side valve cover gasket was leaking oil all over the #7 plug and wire, so I tightened it up - it's not leaking anymore, and I've since changed the plugs and wires, so I don't know if that would be contributing to this still.
Finally, one of the plastic clips that holds the air filter box to the tube where I think the MAF is is broken, but the tubes are connected snugly - but they are probably not air tight, I'm thinking. Could that be causing a noticeable persistent misfire like this? Seems like a bigger problem than that since it's only showing a code for #7, but I'm far from an expert, just trying to turn the wrenches myself to save some dough, but not super experienced at working on newer cars.
Sorry for the wordy post, but I've been trouble shooting this as best I could by myself, and these are all the moving parts that I have thought through based on the great advice I've been diligently reading on this site.
ANY suggestions are welcome, this thing is driving me nuts & I think my wife is going to divorce me if I don't stop troubleshooting this out loud around the house.
Thanks in advance!

 

Additional Information: I noticed this morning that the muffler on the drivers side (under the start of the rear door) is totally shot and was dripping condensation out of the bottom of it. I didn't get a very good look cause the windchill is -9 here this morning, but could this be part of the misfire problem? It seems like a long shot, but I know #7 is on the drivers side, as is this... I will replace the muffler regardless at some point, but I am trying to deal with one issue at a time, and getting the misfire taken care of is my first priority.
Thanks again in advance to anyone who has any ideas...

 

The flashing CEL indicates a higher level of misfire. Like CAT damage level.

The airbox needs to be fixed, but is most likely not the misfire issue, the MAF is downstream from that leak. It can make MAF dirty.

Once concern with #7 is that it is on a cylinder very near a common leak point in head gaskets. While some condensation is normal out the back,,,,,, A good test is a chemical one that tests the coolant for exhaust gases. About $60 at parts store, does maybe 15 tests, some stores rent them. Hope you are not having coolant loss, noise like water bubbles rushing thru pipes under dash.

 

Here's pages from the shop manual, called the RAVE, free download links below. Tranny has heat sensor, would think operation would be questionable these days. Perhaps low fluid.

 

Thank you for the information Savannah - I will look into checking for the head gasket failure... hope that's not the case...(!)
First off, I'm assuming that CEL is CHECK engine light? This says Service Engine Soon - is there a difference?
I have not noticed any coolant loss - i have owned this for a month now and it is not showing low... and no noise of bubbles or anything - I had my ear right by the cap where the coolant is the other day right after I shut it off, and I heard some very faint noise there, like air escaping or something. I know assumptions can be dangerous but I assumed it was normal since it's a plastic cap on a plastic reservoir, and like I say it was a very faint noise.

There was a pretty bad leak of oil around the valve cover gasket right above #7 cylinder... I am not familiar with these motors, but isn't there an O2 sensor right around there too? I was wondering if maybe some oil got on that or the wires leading to it or something and that might be causing this problem too? There was quite a bit of oil on the headers, valve cover, and the plug wires were absolutely coated on that side with oil from this valve cover leak. I tightened it down and it hasn't leaked any since, but I'm wondering if some damage was done.

That sounds like good news on the gearbox, I would bet it's that sensor... it went off when I was driving at highway speed and it was below zero out, so I didn't think it was anything - external at least - that was causing heat... it has been COLD here.
Thanks again!