Conor has covered most of what i was going to say. I suspect a slow leak on the reg but it is very rare. I can't really think of anything else apart from a dodgy firing valve that would give the symptoms you describe.
Not sure if you're comfortable about stripping the rifle and it's a pity your'e so far away as i would have sorted it for you as i have reg internal spares and a reg checker...also firing valves. Conor already mentioned it but there were some early models that had a machined bevel in the block for the firing valve to fit. the newer versions are flat at the block and rely on a thin o ring pressed into the firing valve to provide the seal.The old firing valves had no o ring.