You could try roughing the inside of the pulleys with some emery as they go very smooth over the years.

HOWEVER, i had the same problem on mine, which ended up being a result of a leaking the brake servo.

The servo was causing an air leak on the engine, and then when the alternator started to get loaded up with things like headlights, rad fan etc, the engine would struggle to idle because of the load the alternator presented and because of the air leak it would start hunting, which then led the alternator to slow, and when the engine was revved the alternator would try to stall because of the load and eventually the belt would slip once, and its a down ward spiral from there.

Each time the belt slips and squeals, it wears, becomes looser, etc etc.

Since fixing the running fault i have not changed it since. I went through 3 belts in 3.5k miles.

If you have cold idling problems and alike running problems it could be a similar thing.