Fannie Mae says it has discovered significant operational problems at one of the largest subprime mortgage servicers, Fairbanks Capital Corp. But officials at the government-sponsored enterprise are downplaying the notion that the findings point to a weakness in its oversight. In a letter sent last week to Sen. Paul Sarbanes, D-MD, Fannie Mae Chairman and CEO Franklin Raines said that a review of Fairbanks’ business practices, launched