Arrowhead sent me a pm a lil while back with what sounds like exactly the same issue. It happened to him 1st but we both wrote it off as he thought he may have overtightened the belt. Now that I had the same issue we can rule out "tensioner vs no tensioner". Also I can rule out my ac compressor alignment. Thats what made me look toward the alt pulley as suspect. If it happens again I will definately be scratchin my head but I will find the culprit, especially with the help of everyone on this forum
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On another topic. The optimal position for a tensioner is right after the crank pulley to pick up the slack of the stretching belt except of course when the belt goes directly to a reverse driven waterpump where that would be impossible. The crank is the "driving" pulley and all the others are "driven". If you watch the tensioner on any serp system you can see the tensioner swing in and out with the load of the accessories. Without the tensioner the belt slaps around. If its going up a short distance to the waterpump like my 1st system its not a problem but going all the way to the ac compressor I thought would be an issue so thats why I installed it there.
Ive seen tensioners in other places, like between the ac and alternator but most of the system I checked out had them after the crank.