Stall During Idle When Lights Are On

[whiplash willy]

My SR runs pretty well, except for when I have my headlights on. When they are on, and let off the throttle, as I pull it out of gear and coast to a stop, my idle will drop to around 450rpm and the car stumbles until a few seconds after I come to a stop. Then the idle goes back up to 750rpm.

My engine is a 1998 S13 SR20DET Type-X engine. It is completely stock except for the exhaust. I have timing set to 15degrees, and the idle to 750rpm. I have tried adjusting the idle to 900, 1000, 1100, and 1200 in timing mode, but the idle goes back to 750rpm once the ECU is back in normal mode. The voltage to the battery while the car is running is 14.2, with the lights on and off. Anyone know what could cause this?