I had a 98 ss i.e. which on 18k miles had this problem.
the local shop did the chemical treatment which did not work.
I then had the tank brazed which never gave any problems- i would recommend this option.
Then the idiots at the local shop tie wrapped the breather tube to the frame causing it to implode whilst in for service!!!!).
I managed to get a brand new tank from ebay for £90!!!
Jamie