Take soapy water and dab it all over the regulator to tank conection and around the tank valve steym. Look for bubbles. If You don't find any dab all around the regulator and flow guage, and over all the hose conections. If You still havn't found it pull the hose off the output side of the gas selenoid, and wipe a soap bubble over the end of the fitting, and see if it grows. The pressure drop is pretty common, most ignore it unless it is pretty bad [figuring that You have fixed anything You can]. Turn the tank valve off when You are not welding for any length of time.
Some regulators are self bleeding, meaning that when the bottle is shut off the gas will bleed out of the regulator so as to relief the pressure on the bordon tubes etc, this is done so that the reg will remain accurate. Normally this would bleed off both sides of a two stage reg but this may not be neccesary on a flow meter????