Richard is probaby right, but I am not mechanically inclined so I will need to rely on my mechanic to check everything and try to get to the root of the problems.
Until then, I have resorted to Tony's methods for circumventing the start/stall problem. On my car, usually it runs smoothly after I get it going and prevents it from stalling. My car stalls out with cold and hot engine.
I forgot to mention in my last message that my mechanic replaced the idle control valve already and it didn't help at all. After that he replaced the air flow potentiometer assembly. I now believe what Richard is saying: probably neither part needed replacing.
I now detect "vacuum" problem only in 5th gear at 3500 rpm when I try to punch the accelerator. There seems to be no problem above 4000 rpm.
Since I don't work on the car myself, I tend to find a way to live with the problems instead of taking it to the shop every time.