Clinton testify to grand jury Aug. 17, 1998.
And:
While under oath, Clinton denied having had "a sexual affair," "sexual relations," or "a sexual relationship" with Lewinsky, and on January 26, 1998 claimed "I did not have sexual relations with that woman, Miss Lewinsky" in a nationally televised White House news conference.

Blue dress:
The Blue Dress - Why Clinton Has to Resign
An editorial by Lars Poulsen - 14 September 1998
"After 6 months of anxious waiting for the DNA report from FBI, we could finally read on the Internet that Monica wasn't bluffing: She really had secured a memoir of "the Big Guy". But while this may have been what most of the readers were looking for, there was something much worse in there: A good case for perjury. "

Another source:
# On 1/21, the day the story broke in the Washington Post, Newsweek goes on AOL with excerpts from the Tripp tapes. In its account Newsweek reports Lewinsky "says that Clinton gave her a dress."

# On 1/21, the Drudge Report wrote "investigators have become convinced that there may be a DNA trail that could confirm Clinton's sexual involvement with Lewinsky. Tripp has shared with investigators a conversation where Lewinsky allegedly confided that she kept a garment with Clinton's semen on it -- a garment she allegedly said she would never wash."

# On 1/22, Sam Donaldson on Good Morning America announced that in his Paula Jones deposition Clinton denied he had a sexual relationship with Lewinsky, but "apparently acknowledged giving her gifts, including a dress." That same day the Washington Post reports that Starr is "searching for gifts that might show whether there was a relationship, including reports that Clinton gave Lewinsky a dress."

# On 1/23, ABC first reported a "semen-stained" dress. "According to a source, Lewinsky says she saved--apparently as a kind of souvenir -- a navy blue dress with the president's semen stain on it."

# On 1/24, the New York Times reported: "Investigators who have heard the tapes said Ms. Lewinsky made references to gifts she had received from President Clinton, including a dress. On one of the tapes, the investigators said, Ms. Lewinsky tells a friend, Linda R. Tripp that the dress contains a semen stain from President Clinton."
http://www.concernedjournalists.org/clinton-lewinsky-story-how-accurate-how-fair-blue-dress

So anytime after say, Jan 1998, if anyone was following the news, they would have at least suspected that Clinton lied under oath. And by mid Sept 1998 would known with certainty that Clinton had lied under oath.

In your post:
"Sanford told The Post and Courier in September 1998"
and:
" Larry Craig, asked by the late Tim Russert on a Jan. 24, 1999, episode of "Meet the Press" whether he thought Clinton should resign"
And the other times you provide are well after mid Sept 1998