This is just a suggestion, but have you tried turning the engine over by hand. It might point to a spot that is tight or you might hear some kind of unusual sound in a certain location. The old rule was in checking belt tension pushing down with thumb should have approximately a half-inch of slack. It is not exacting as getting a tensioner from a parts house to verify the exact measurement. Lastly, it is possible you have a defective belt there, even though it is a high quality brand of belt??