If I let the car sit for 2 day, it will do it. Also, I find something really weird is that if I turn on the A/C when engine just start to get warm(about 70C or so) and turn on A/C immediatly, the idle will not drop at stop lights at all. But if I let the engine heat up to normal working temp(usually about 90C), and turn on the A/C, idle would drop. The problem usually disappear if I keep drive it for a day and after......but still, would happen again once in a while, if the engine feels like to.......

Lee, what's the best guess??

Andy Kuo
'92 400SE
71,118 KM