hot start surge

i noticed that whenever the car is hot enough for the fan to come on, it will surge when i start it. other than that the idle is perfect. doesnt surge or hunt at all. i think it is related to the fan and charging system. as soon as i hear the fan the volt gauge drops down and goes back up a little. the gauge even goes down when i roll down my window. i have underdrive pulleys and i was thinking about getting an overdrive pulley for the alternator. anyone ever use one? my first thought was to upgrade the alternator to a higher output but i guess i could try the pulley or even go back to the stockers. what do you guys think?

btw, the alternator charges fine under load.

i just wanted to add that if i set the idle up at about 900 rpms it will start up and usually settle within a second or two. if its any lower it will surge and die(only when its hot). if i hold the throttle or drive the car right away it will stop surging. this did not just start to happen. i just wanted to get some other opinions. i guess im going to swap to an overdrive pulley and see if it helps.