I have an AGA that heats a hot water cylinder indirectly. There are two towel rails also on the circuit. I installed the system about 12 years ago and put a new cylinder in about 5 years ago. It relies on gravity. It's always worked well except, occasionally, it gets an airlock. This occurs on a random basis and it has sometimes gone for a couple of years with no airlock. Recently the circulation stopped so I assumed there was an airlock and cleared it. It worked fine for a week and then locked again and won't start circulating. Both flow and return are piping hot almost all the way to the cylinder. It is as if there was a block in the core within the cylinder but I have run water through that. What can be going wrong?