Make sure it is the correct belt with 6 ribs. Has anthing been replaced or worked on lately? Fan clutch, AC compressor, power steering or alternator? If so, the wrong pulley may have been used or put on incorrectly. The belt tensioner failing could also cause improper alignment. You might be able to just look in the engine compartment and see if the outer edges of the pullies line up or feel if they are loose. The fan clutch has 4 10mm nuts that could be coming loose and that would allow the center pulley to be out of alignment. Or stop at any auto shop and tell them the symptoms - often they will look and at least tell you what they think and a cost and then you have additional information.