boom from New Holland W170 going down while running machine

Run a NH W170 with around 1000 hrs, since a while the boom is going down slowly when I use the loader (from the top to ground 30-40 seconds), is there something wrong with the boom suspension or a leaky cylinder (that's what the dealer sais) ? When you raise the boom 100%, it will stay in that position and will not leak down ??????? any idea what this could be? Thanks