Suggest you update this thread with the diagnosis by the gunsmith, it might help others.

A potential issue could be the cylinder indexing not working as well as it should. I realize the pistol is all parceled up now, but for future reference (e.g. when it is returned to you) you could check that out. With NO C02 bulb in the pistol and no pellets loaded, indexing the cylinder (with the hammer), should cause the cylinder/shells to line up precisely with the barrel (this can be viewed with a torch down the barrel - NO C02 ).

Some of the Umerex 8 shot rotary magazine pistols can succumb to a fault whereby the magazine/pellet do not line precisely with the barrel, result is the first few shots are good, the following shots go 'phut' as the pellet strikes the edge of the barrel. Seems feasible that this type of problem might occur on any indexing cylinder gun.