I still don't fully understand. So you're saying that the regulator slowly stops sending gas to the keg for no apparent reason, and then the keg loses all of it's pressure? And this only happens if you turn the pressure down? If so, it sounds like two separate problems, a leak, and either a faulty regulator or a process issue. What brand of regulator is it? When you change pressure are you adjusting up to the desired pressure, or trying to adjust down from a higher pressure?
Some newer fancy regulators will vent the excess pressure when you simply lower the pressure knob so that it will give you an accurate reading, but most do not. For lowering the pressure on most regulators you need to turn the pressure way down, vent the keg, and then adjust up to the desired pressure. If you have a bad leak and try to adjust the pressure down to 4 psi without venting, it could look like you're adjusting the pressure to 4, when you're actually adjusting it to 0, and then the gas in the keg and lines escapes through the leak.