Normal city-style driving - ML drives great - smooth idle but with only a slight rocking when stationary, smooth acceleration - no problems.
Driving fast, then suddenly push the accelerator pedal to the floor and get the revs up to around 5000, then I get the problem.....

Problem is that cylinder #6 is fails and ML idle is quite rough - i.e. disconnect the #6 cyl spark leads gives NO change in idle.
Turning the engine off and waiting for several seconds then starting again - engine idle is back to smooth idle again.
Fixes tried: new plugs, new plug leads, new coil pack, new air-flow sensor (sensor in the air filter hose between filter housing and manifold), new crankcase sensor.

Diagnostic tests were run (did not note error codes !!!) and all referred to cyl #6 and potential damage to cat. conv.

Engine has 107,000km. I took ownership at 106,000km's.
Cat converters were removed and welded around the pipe seams when car went in to MB (Melbourne)for 100k service - it was then that service people noticed cat. conv. seams were cracked and decided to weld them.

PLEASE, PLEASE, any ideas.... could it be a faulty/blocked cat converter? or faulty control unit? something else?

Andrew....