sohcvtec1995

My ex coupe is having some kind of voltage problem. When it is cold outside my greddy timer reads out like 12.3 and when i stop sometimes it drops to like 11.9. and it is at night and it is worse when i put the brakes on. all of the lights dim. But it is only at night when it is a bit cold out. I have underdrive pulleys could this be the cause of this problem? just started like two weeks ago.