Have you tried a different mag?
I had a similar problem crop up on a HiCapa, changed to another mag (both were stock) and the problem disappeared. The original mag was toast, I suspect (though I hadn't looked at it further) that the valve would stick at a certain point, or the ports were dirty.
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Ár skal r?*sa, sá er annars vill
fé eða fjör hafa. Sjaldan liggjandi úlfur
lær um getur né sofandi maður sigur.
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