Benjamin Strange Marauder Accuracy Issue

I've had an unregulated gen 2 Marauder .22 (Lothar Walther barrel) for two years now. I've put around 750-850 rounds through it - and until a few weeks ago, accuracy improved from reasonably solid to almost amazing since I bought it new. A few weeks ago, it started throwing one out of every 6-8 shots very close to 5 inches directly below the other 5-7 shots at 25 yards - and I can cover all of the other 5-7 shots with a dime (with those shots being centered on my aiming point). The one flier is always very close to 5 inches directly below the other shots and the other shots are always a very tight group where I would expect them to be. Could anyone offer any thoughts on what might be causing this?
 
My first thought is chrono it and see if it's speed/valve related if that's good clean barrel. Or even clean barrel first if you don't have a chrono. I'm learning these things so I'm sure there's other opinions.
Thanks Cam. I did recently clean the barrel and have shot it over a chrono just before it started doing this. I haven't got a chrono reading on a shot that went low though. The gun holds air perfectly at any pressure (not even a slow creep down in pressure if it sits for a few weeks). Was wondering if it could be an issue with the hammer or the valve/spring. It's just weird how all but one shot in a given group are tightly grouped and right on target and the one flier is always very close to 5 inches directly below that great group.
 
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Did you clean just before this accuracy aberration or after?

Until you run it over a chronograph it is all guessing.
I cleaned it just after it started having these strange flier exceptions, but that didn't make it better or worse - still having the same problem after the barrel cleaning. I plan to have another chrono session ASAP though. I want to.see how the velocity on the low fliers compares to the velocity of all the rest of the shots in the group.
 
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I have had mine only a few months, but the factory mag that came with it never worked correctly. The spring comes out its hole when I put more than a few in the mag.

One of my Prod mags tends to to deform skirts. I would try single loading and see.

Is the hammer traveling freely? If it gets dirty it’s travel could be inhibited in some way.
Does the low shot sound different?
 
if one of the "holes" or pellets was not lined up right it could be clipping the loading ramp as it rotated,I could be way of course,it just a thought.The probe pushing the pellet that is not centered,just single load it and see if the 5th shoot is still off.:unsure:
I haven't tried it single shot - will do that at my next shooting session. Thanks!
 
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I have had mine only a few months, but the factory mag that came with it never worked correctly. The spring comes out its hole when I put more than a few in the mag.

One of my Prod mags tends to to deform skirts. I would try single loading and see.

Is the hammer traveling freely? If it gets dirty it’s travel could be inhibited in some way.
Does the low shot sound different?
Will definitely try it single shot and check my mags closely. I'll also check the hammer/travel and make sure all is OK there. It just sounds like a bit of a weaker shot (not quite as loud) when it drops a shot low. Thank you for the input!
 
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I had a magazine that was not rotating to the center of the barrel and would throw a flier randomly. I cleaned the mag with some balistol and a q-tip problem solved.
Thank you for the input. There's definitely enough concern here on magazines to prompt me to clean and check all three of my mags. I will also smooth up and chamfer edges that the pellets come in contact with a small round file and sand-paper before cleaning them.
 
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