Silicon tubing will lose CO2, more than CO2-resistant anyway. But if your pressure is dropping quicky, then wouldn't the leak have to be before the regulator, since the cylinder wouldn't release more CO2 if there was a leak after what is stopping it, if that makes sense. However, a leak would effect the amount of CO2 that is going into your tank....
How is your regulator attached to your cylinder? I know that it has to be very tight to work, and I used teflon tape as well at that connection.