Does your charger have an "equalize" botton on its panel, its possible that the cells are at different states of charge, its a hail mary from the sounds of things but batteries are expensive and you cant say you didnt try ,if it has one push it and walk away.
A light should come on to indicate "equalize". The battery needs to cycle fully (approx 8hrs, make sure your water levels are topped up).How do your battery cables look , any strange looking bulges under the insulation (especially the Pos. cable).