Do you perhaps have it too close to a radiator? Regards the watering, I think a jug once a week at this time of year might be too much....instead of watering once a week whatever, try letting the surface dry out before you water your plant. Also, make sure it isn't sitting in water - let it drain fully before putting it back on it's dish or whatever. Do remember that it will loose leaves at some point wahtever you do - even evergreen leaves don't last for ever.