Land Rover Repair: Freelander K series misfire, rotor coil, lambda sensor

Question
Freelander 1.8 K series (petrol) 1998 with 108,000 miles, Head gasket changed with the thermostat convertion, at 105,000 miles.
vehicle works perfect?? for about 10 miles then the misfire starts.
1. If I switch the engine off and back on again it cures the misfire for a mile or two then it comes back, so I repeat this until i get home??.
2. I have changed the plugs & leads, dissy cap & rotor,coil, crank sensor, lambda sensor, throttle body, taken fuel pump out of tank cleaned put back (no foreiners in pump or tank).
3. had test book on it and it comes up with no faults found?????????.
  HELP  

Answer
Hi Graham,

could be exhaust valve sticking or even exhaust system itself.  I know that Series II engines are known to have the sticking exhaust valve problem that robs them of power and misfiring engine.  Maybe the same problem exists in your model.

Only cure is a complete valve job - clean or replacement.  No additives can help on the exhaust side of your engine...it just burns itself up.  

It could also be an exhaust system problem but I think you'd know this...the sound would be much deeper than normal.

Misfires are usually fuel feed or exhaust related.  I'd guess its on the exhaust side...since you've taken care of the fuel feed side of the system.

Best of luck.

JohnMc