This is a new one. What does it look like when its melted. Maybe its crushed. Are you replacing it with the proper gaskets? I would also check the seat in the bottom. If it has a crack or groove in it, you may simply be over tightening it and causing it to fail, when the problem is the seat needs to be replaced. Shine a flashlight down into the barrel and see if its got a problem. You can also reach your finger down in and feel the surface of the seat to see if its bumpy.