Something that could happen suddenly (like during 30 minutes) could be a dry fire (not good for springers). I'm pretty sure I ruined the accuracy on a Walther Terrus by shooting pellets that were too light for it (5.56 grain). I'm guessing the spring on my Terrus broke, although, it was not obvious to me at the time. I returned the rifle to the store so I have no ability to tear it down and check my theory. Dry fire is much akin to shooting too light pellets.Have you previously done a Chrony test on the rifle?If so, retest and see if you notice significant velocity drop.