The same thing happened on my 99 GTV 2.0 TS about 2 years ago. It happened twice. The revs just dropped and the car kept stalling (didn't matter if the engine was cold or hot). The problem happened once then 2 or 3 days later it happened again and has never re-appeared.
I don't know much about engines but I wonder if this is an early indication of Lambda Probe failure. I have seen somewhere that if a Lambda probe fails the engine goes into "limp home" mode. Maybe I have been running for 2 years with a bad Lambda!!
The engine lost its smoothness months ago. The lambda has been diagnosed faulty and is being changed in the next couple of weeks.
My symptoms were:
Car stationary, in neutral, engine warm, try and hold the revs at 1800 RPM, then build the revs up to 2000 RPM slowly (try this 3 or four times). My engine used to stutter/vibrate slightly and the revs would hunt between 700 RPM and 2000RPM before settling.
Hope this helps.