Symptom: After first round of fixing in detailed in Wonky Idle I, the same thing still happened although much less severe.Over the years, there have been many things wrong with my car, some of which I have investigated and fixed.
I meant to post these findings as a reference in the hope that it may help someone (and myself, because after a long time I might not remember it anymore). It seems that some of the problems are not very common (but some are, I think).
Cause: There was a suggestion here on AO that this is due to a bug in the ECU firmware, specifically with regards to fuelling cut during coasting and fuelling restart as the speed dropped below a certain threshold.
Remedy: Upgrade ECU firmware. If it is not possible to get official firmware update, look for tuner that offer ECU flash remap. Make sure the base map is from a newer version.
After updating my ECU firmware, the engine runs perfect except in one kind of condition. On cold start where the engine is still not up to temperature and the A/C is running, when coming to a stop, I can feel the clutch disengaging and the engine speed will drop faster than normal. However, it never dropped to the point where it feels like stalling so I think that this is acceptable although not perfect. This does not happen if A/C is not running or if the engine has warmed up.