Anyway, spoke to an informative techie at mitsubishi George - who gave me quite an interesting bit of information.
He reckons that the EGR diaphragm does not really ever close at JHB altitude... due to the lack of air pressure. Accordingly, some deposits can form over time, and when you come down to the coast you can get a situation where the diaphragm tries to close, but cannot. This gives a rought then smooth, then rough then smooth sound to the enige when the throttle is pushed down moderately.
Apparently this just needs to be cleaned (the same diagnosis and solution given by 4ePikanini when I asked him about my experience). Driving with it (and avoiding it by accelerating smoothly), does not seem to pose a risk if you cant clean out the EGR immediately.
