NGLI 3 seems a little stiff. Is that what is called for by the manufacturer? It sounds like what you need is a highly water resistant grease that has good tackiness qualities. Sounds like you should look for a grease with good adhesive and cohesive grease. The adhesive properties are that it sticks to everything, and cohesive properties which mean it's a stringy grease not wanting to let go of it's self. Both properties make for a grease that isn't easily removed or slung out which prevents lubrication loss. I would suggest either
Paragon 3000 or
880 Crown and Chassis grease. Both are highly water resistant and both have a stringy texture. However I would recommend a NGLI 2 in place of a NGLI 3. The harder grease may be part of your problem in this application. Seeing as this is probably a very dirty environment added to the stiff grease it could possibly be caking up to quickly and simply breaking apart.