Ah, then I would look at the internals. I.e. hammer/trigger/sear combo. The hammer isn't getting caught by the disconector/sear and being held to the rear. It goes forward as the next round is loaded and fires the gun again on closing. The bolt works again, but the third time there may not be enough energy to fire a thrid time. The hammer comes to rest in the forward position. Obviously the rifle stops working unless the hammer gets cocked again, by pulling the bolt back.
Alternatively, the hammer/sear engagement may be worn and when the bolt slams forward, the vibration lets the hammer fly forward. Had that happen with an FAL with US internals. Weak US spring wouldn't hold the sear in place.
It would look more like worn internals (or someone tried to do some home gunsmithing on it before you got it) or a weak spring (which is less likely).
You can use US parts to ditch the thumbhole in the same manner as any AK. You'll need whatever the number of parts required is. You can't use mag parts as there aren't any .223 mag parts made in the US. Between the US made internals (same as 7.62x39 for Mak-90) and US made furniture (again same as Mak-90) you should be able to do it pretty easy. I'm not sure about the gas piston. I think the .223 is different from the 7.62x39.
I have a Choate thumbhole on mine. It looks tacky, but it fits, is comfortable and handles great. For looks I have two Hungarians and a Bulgarian. My Mak-90 .223 is a working gun. No cares about looks, all about function.
Ross