Q: idle speed too low on 97' 626 4cyl auto

hi, there
I am trying to tune up my 97' 626 4cyl auto. However, the idle speed can not be adjusted to the manual range (650-750rpm). The highest I can reach is barely 655rpm by adjusting the idle speed screw. When I drive on the road at the red light, with AC on, the idle speed often drops to 500rpm. What's the problem? How can I fix it? Also, the ignition timing can turned to 0(+-)1, but not very static. Thanks for your suggestions.
Have a good day.
zl2k

Let me know what it turns out to be.