Land Rover Repair: Loses power after warmup, random power, carbon buildup

Question
I have a 96 Disco which will lose power for about 3 seconds then run normally for about 5 seconds then repeat the cycle again and again. It starts doing it after it warms up and at speeds around 40 miles per hour and higher, if I slow under 40 it doesn't seems not to do it anymore.  After a couple of days the check engine light came on and the diagnosis was a misfire of the #7 cylinder.  I changed that coil (#4 and #7) and it still does it.  I spoke with a Land Rover tech rep and he said something about a speed sensor malfunction.  Does that make sense or do you know of anything else it might be?

Thanks

Answer
Hi Glenn,

Does your '96 Disco have the 4.0 engine or the 3.9?  4.0 engines have problems with sticking exhaust valves.  It complicates the mixture and can cause all sorts of mis-diagnosis.  The first indications are misfires and random power loss, especially when the engine gets warmed up or on hot humid days.  Valve job is the only fix that I know of.  Pouring valve cleaner does not work because the culprit is with the exhaust valves and carbon buildup causing sticking.  

Does not seem to affect the 3.9l engines. I have not yet experienced this problem (knock on wood).  

Let me know what engine you have and we'll go from there.

Regards,
JohnMc