I haven't got a clue. After trying everything recommended in every PS tutorial and every forum, and still without success, a conversation from the movie "Top Secret" comes to mind.
The manager, who was supposed to free an American musician from an East-German prison, visits him in his cell and says with a mournful face:
"Nick, I have tried everything. The embassy, the consulate..."
"Don't worry, there will be a solution."
"...the President, the CIA...."
"Don't worry, there will be a solution."
"... even the Marines. But I still can't make my wife have an orgasm."
"I told you not to worry. Here's the solution", the musician says and takes a huge box from under his bunk. The inscription reads: THE MIGHTY ANAL INTRUDER.
Anyway, the bottomline is that all jpgs with banding are fucked. No remedy.