Had a similar issue in the past so far the only thing that managed to work for me was something called HCL89.
It had managed to clean my tiles and restore them to their original self
One good thing i like about it is that creates a protection layer that helps prevent my tiles from getting too overly dirty
I usually get it from http://hcl89.byethost10.com or http://hcl89.atwebpages.com
Hope this helps you out