I'd try a phosphate remover for a day or two and see if that helps. Just, do it long enough to get close to 0 and see if it starts creeping back up. If it does you know it's not just wash out from where your removed the bio-balls. If you have a dsb and have been feeding heavy it may be poluted with phospates as well. What have you been feeding how often etc.
hth
chris