Strange M-rod Intermittent Flier Issue

[xring308]

I've had an unregulated .22 cal gen 2 Benjamin Marauder with the Lothar Walther barrel for 2 years (purchased new). It was easy to tune and has had solid accuracy since I initially tuned it and found the pellets it likes best (JSB Exact Jumbo Heavy 18.13 gr and Hades in 15.89 gr).  Everythong intside a quarter normally at 50 yards.  It's been flawless ever since but in my past few range sessions, I've been having one or two fliers per group that are around 10 inches (!) low - with the rest of the group being dead on target and tightly grouped. It's not happening with every group though - maybe in one out of every 3 groups I shoot. The gun holds air pressure perfectly - no leaks at all when not in use. Has anyone had any experience with this problem? Thanks in advance for any advice!

[JPSAXNC]

It sounds like there might be some debris interfering with the hammer stroke or valve stem. Maybe take it apart and clean everything.

[Yogi]

Get a chronograph and see if your velocity drops on those fliers!

-Yogi

[mobilehomer]

If you are tempted to lubricate the hammer - DO NOT USE OIL/GREASE OR ANY "LIQUID" type lubricant. A LIGHT, burnished in dry lubricant only. Moly powder or tungsten disulfide are best.