gas welding regulator problem,..

Hello, I am doing some cutting with an older victor oxy/acetalene set up. This went as expected for a few days but I noticed a kind of
chuffing or squeaking sound from the oxygen regulator this afternoon. If the regulator is set for 25psi for example the pressure will range rapidly between 24-26. You can see the cutting flame shorten and lengthen in time to the sound. The oxy tank shows 1500psi or about half full. I changed tips but no difference. If I turn the torch off and leave the tank open the regulator stays dead on 25 with no change. It's only when the oxy torch valves are open that the problem presents itself. Any idea what would do this? It's a victor setup with not much use on it. Thanks for your time.
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