If I understand you, the screw is ok, the little pin that moves the bar is ok, not stripped out, but is the little rubber piece that it goes through there? I don't think that would keep it from turning backwards, but it helps it from coming out if its out of the tension-pin.
The only thing I can think of is that you're maybe bottoming out the bolts, or they aren't getting enough tension on the bar itself, allowing it to loosen while you're sawing.
You might try asking this question in the chainsaw forum, but since you're in the milling forum, I'll say this; I have to keep tensioning my chain on my 394 after every few cuts; milling puts much more than average stress on a chain, and even a "broken in" chain is going to stretch a bit. It just slows down stretching so much after the first couple uses, in my experience, IME.
Also, I've broken at least a couple tensioners by trying to keep the chain a little too tight when starting a big slab, as by the end I was expecting it to be a little loose. Bad idea. I now keep an extra tensioner in my saw tackle box for this very reason. Being without a tensioner in the woods is a PITA, and makes me want to break stuff that isn't already broken.
