I would still suspect the connection between the regulator and CO2 cylinder. That nut has to be very tight, so you are compressing the hard nylon or fiber washer to act as a seal. If you use a Permaseal,
The Best Aquarium Regulator & CO2 Parts, the connection doesn't need to be that tight. I can't use those seals on my setup, because there is a grove in the face of the cylinder half of the connection that is in the way of the seal in the Permaseal, but if I could use it I would be doing so, for sure.