A Gates V belt should be as close to perfect as makes no difference! An earlier post suggested warming it to ease the storage deflections out of it. I'd do it in a bucket of very hot water, not coiled, and then get it onto the machine as fast as possible while it is till warm and run it at medium speed, good tension, off load for an hour. If it is still causing bother after that, take it back to the shop. You could try sliding it manually along the V of a pulley and see if there are any obvious high spots, or carefully and lightly caress it with a stick of chalk while it is running to find a high spot. Just hope you don't find a high or low spot on one of your pulleys when you do this.