rough idle

i have a 1998 crown vic police intercertor. sometimes the car runs fine but most of the time it runs rough. When running rough the volt meter is very low until you give it gas. does not want to stay running when you start it no matter if it is warm or cold. need some help to try to find out why the idle is low and the volts are low. there is no rhyme or reason on when the car runs normal. Is there a way to adjust the idle or is that regulated through the computer.

The idle is computer controlled via the Idle Air Control solenoid (IAC).

I suspect that the root of your concern is voltage, although it could be that the IAC cannot keep the enigne speed high enough for the alternator to charge effectively.

If the idle smooths out when you increase engine RPM with your foot, I would just replace the IAC. It is an easy 5 minute job that virtually anyone can do. I suggest that you use only OE parts, as some of the available aftermarket parts are not of good quality.