Same thing happens to me. I actually just posted on a similar topic too about this.
my car idles real low and shakes my car when im at stop lights while holding down the brakes (i'm not pressing hard on them either). when i roll down the windows, i get the same effect. i have a sub woofer in the trunk, 12inch 800 watt, and hits hard, but it doesnt make the rpms drop unless i crank it so loud it hurts my ears. but with the "brakes" problem, i just put it in neutral while at stop lights and my rpms stay fine around 780-950rpm.
GL, i hope both of our problems get solved
1999 Auto V6 3.8L 35th Anniversary
CAI + True Duals Exhaust
17inch SVT rims, mach1grille delete, 8inch shorty antenna, 35% tint
hopefully's: Tuner, UDP's, possibly 4.10 Gears, lighter driveshaft