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Do you have the vz.58s 15" barrel or the cz858 with the 18.5" barrel?
I ask this because I'm going to guess that the 15" version has the selector switch that switches to full auto but acts as a safety because mine does this too.. Also the trigger group welds are different between the two models where welds are placed forward the reciever on the shorty and towards the rear on the long barrelled version possibly making the selector act differently.