Thanks for your help on this @Wolferl,

It is possible the tube goes into oscillation due to a misbehaving capacitor, called “the compensation capacitor”. Normally all A3 units have it set for 150pF, linked on the type of Geiger tube used (SI29BG), but in this case the value should be bigger to prevent these oscillations that result in high counts.

Either this, or the tube is faulty going in avalanche (unlikely).

You can either send the unit back so I can have a look at it, or change the capacitor yourself based on my indications. I’ll be happy to accommodate any of the two options.