I've had this problem for months now.. I dont know if the regulator in my alternator is bad or what but I figured Id ask before I go on that assumption. so.. when I drive its fine or coasting its fine but when Im under 1000 rpm the voltage goes to like >10volts. During the day its bearable but at night with the headlights and trying to use turn signals its aweful. Sometimes when I first start it I have a hard time with it too, itll stall out. Id like to be using my air but that will be constantly two footing it..

 

check the stator

 

A diode or two in the alternator may be bad, but the battery may also be the culprit. Does it get hot during a drive?

 

actually the battery is new, i replaced it over the winter and i had the same problem when the other battery was in it. Im guessing the diode thing too, just gonna have to take it to be tested.

 

A couple of more thoughts.:
The idle speed shouldn't be affected by inadequate charging. You could have additional problems with the engine controls.
You haven't installed underdrive pulleys btw.? If it really can't keep up below 1000rpm then the alt. is either underdriven, overloaded, or has lost one (or even two) of it's three phases (diodes).
The voltmeters in these cars are not reputed for their accuracy, but problems with the turn signals does indicate that the voltage gets quite low..

 

no underdrive pulleys, gonna have to have the diodes checked i guess

 

Just replaced my alternator and problem solved.. I tested it at work and it passed everything but the Lamp On Test? no idea what that means but it was supposed to be 13 point something volts but it was 11v.