if you offset your loop, when you get it right stick a set of needle nose pliers in there and tighten up that loop and it won't slip. usually all you'll need is a half twist in the bottom of your string if you're going to do it that way.

a lot of this is weather changes. believe it or not 8125 will change a little bit with the weather, and the other cheaper materials are worse. its why i advise hunters to put 452x on their bows, even though its just a few fps slower. it is remarkably stable in all types of weather/humidity.

one thing i want to say about peeps, besides i don't shoot one (no-peep for me), is that some real good shooters i know put a drop of liquid paper on the side of their peep where the string goes. this way they can tell if the peep has ever moved, because the liquid paper will crack right there if it has. like nubo said somewhere, depending on where you have the peep in relation to the twist, will alter the way it rotates or doesn't. so if its moved that could be your culprit. another thing is depending on how you have it tied in, that serving will eventually want to slide away from the peep, and if its been putting pressure on the gap around your peep and moves, your peeps rotation will change. when i put in a peep for myself i set it up to come back properly without being served in, but that's me. then i serve it in without affecting the gap around it. that way the serving won't be backing off and effecting it. or you can know that and just push your serving back down and make sure its right before climbing the tree.

maybe any of that will help, i don't know.

or your string could have stretched.