[v8volvo]

That is an odd one.

One of mine went through a couple belts after I replaced the motor with one that had sat for a while and had rust on the crank and alt pulleys.... the rust wore the belts down quickly, but now that the rust is gone the 3rd belt is lasting. Doesn't sound like your issue, though.

Alternator bearings OK? Are the rubber bushings that mount the alt in its bracket in solid shape? Those often wear out and cause the alt to sit crooked, which can be hard on the belt.

You might want to put up a picture of what your setup looks like -- all of my experience is with the US style setup, which has the alternator mounted down low on the right side of the car, above the oil filter. If yours is the later Euro-spec style with the alt mounted up high on the left side (where the power steering pump is on our cars), then I'm afraid my help is not worth much...

Any weird noises, or does this just happen quietly? Is the crank pulley/harmonic balancer in OK shape? The balancer is a 2-piece design with a layer of rubber sandwiched in between, and I have seen on other cars where that rubber layer gets old and shrunken and then it makes the crank pulley wobble and throw the belts off... worth checking to see if the 2 halves of the unit can move or if it looks like anything is wobbling when the engine is running.