Hmmm, I was reading this thread only yesterday thinking 'what a nightmare' then today, on the motorway (same engine: 2.5 diesel), I was overtaking at just over 75mph, going uphill, giving it maybe 2500-2800 rpm and the engine warning light came on, along with significant loss of power. I put the clutch in immediately, the light went off, and power was available again. I then drove 300 miles, going between 55 and 70mph, with no repeat of the episode (I didn't thrash it again though, lol). I wonder if it's related to your problem?