I broke the Fan/alt belt today - what are you using on your car?

Last 340 Dart I had would roll the alt. belt frequently. Pulley alignment was perfect. Was able to get it to roll by just holding it at about 2k. Had some sort of weird harmonic thing going on. Every so often belt would get in this weird vibration...followed by a roll. For the life of me I can't remember what fixed it...