Dummy computer on idle RPM?

My computer seems to be really a idiot on idling speed, it could start the car at 1, 200 and lower it down to 400 shortly after which may cause car shaking like a misfire (or maybe it does produce misfire then).

So how could I tune the computer? Or is that caused by a bad spark plug?

Thanks in advance!

BTW, I just pushed my stang passing 140, 000 miles, adding 8000 during last 4 months.