hi sounds to me that the pulleys are worn the actual v's are worn at the sides and are more like u's this allows the belts to run on the top edges and therefore wearing them out too quickly, could be wrong but in air conditioning and ventilation systems this is a symptom of worn pulleys, may not be all the pulleys but check each one for wear and change the one you find to be worn,
also when you get belts make sure they are a matched set or at least as identical as you can, even though the numbers are the same the belts will be different sizes
john