Erratic velocities from DAR Gen II 22 cal.

My grandson's Gen 2 22 cal DAR suddenly started to shoot at inconsistent velocities. Sometimes they exit the muzzle with the normal or accustomed sound and proportional velocities but on other occasions the shots have very little power / sound. ( 342 fps Oehler 35 P chronographed with 18 gr. H&N. X - treme ) Could it be a faulty regulator? Could any of you experienced airgunners provide an explanation and possible cause? Thank you in advance for your help.



Luis