It sounds to me like it might be an ammunition-related problem.
If it was a headspace problem, it seems like the first rounds you chambered would have done the same thing.

Are your cartridges reloads? If so, were they full length resized or simply neck resized? I have sometimes encountered a similar problem when neck resizing only. At any rate, get some ammo that is factory loaded. If new ammo causes the same problem, take it to a competent gunsmith for examination.

Clint