Land Rover Repair: 1999 DISCO II SEVERE CYLINDER MISFIRE, valve cover gasket, independent mechanic

Question
I have a 1999 Discovery 2 SD (i think its an sd...)with approx 102,000 mi. So the other day I was driving (at approx 45-55 mph) and when I hit the gas the engine started vibrating heavily and the "service engine soon" light began blinking then went solid. I restarted the engine and didnt notice anything at first then when traveling again at the same speed and I think same gear it started vibrating again and the light blinked again. Had it towed to the dealership (per Land Rover reccomendation) they ran the diagnostic ( found cylinder misfire) said it was a bad plug and most likeley a valve/seal problem which would require what they called "a valve job" (priced at $3,400). As usual they claimd it might just be the plugs, but given the vibration at that speed its probably going to require the valve job and the plugs (price approx 4,000). Called an independent mechanic they said a valve job was probably not needed very unlikely (said usually happens at around 200,000 mi.)but the coils or plugs or both are probably bad. The only thing is I had the 90,000 mile service performed last summer and Ive had cylinder misfire problems with a Jeep Cherokee and Ford Explorer (both with 4.0 liter engines) and I never felt vibrations like I did.... So could a faulty plug and or wire (cylinder misfire) cause such a vibration and make the electronics react like that??  Your thoughts??

Answer
I would have to agree with the dealer on this one . . . your symptoms sound like the sudden onset of sticking valves.  You might want to read the article on sticking valves on the Robison Service web site.

You could try swapping the plug and wire - not much work there - and see what happens.  If the problem persists you'll have to take the next step.  

If you need to go to the next step I'd suggest removing the plenum and valve cover gasket and manually checking for a tight valve.  If you find one, proceed w/ valve job.  If not, swap a coil pack.

We see this situation fairly often on trucks like yours, at 100k miles.