Using your logic:
2007 - no undefeated teams = No National Champion
2006 - only one undefeated team = National Champion = Boise State
2005 - only one undefeated team = National Champion = Texas
2004 - three undefeated teams = Split National Champions = USC, Auburn, UTah
2003 - no undefeated teams = No National Champion
2002 - only one undefeated team = National Champion = Ohio State
2001 - only one undefeated team = National Champion = Miami
2000 - only one undefeated team = National Champion = Oklahoma
1999 - two undefeated teams = Split National Champions = Florida State & Marshall
1998 - two undefeated teams = Split National Champions = Tennessee & Tulane
1997 - two undefeated teams = Split National Champions = Nebraska & Michigan
1996 - no undefeated teams = No National Champion
1995 - one undefeated team, one one-tie team = National Champion = Nebraska (Tulane)
1994 - two undefeated teams, one one-tie team = Split National Champions = Nebraska & Penn State (Texas A&M)
1993 - one undefeated team = National Champion = Auburn (Auburn did not participate in post season competition due to NCAA sanctions)
1992 - one undefeated team, one 3-tie team = National Champion = Alabama (Michigan)
1991 - two undefeated teams = Split National Champions = Miami & Washington
1990 - one one-tie team = National Champion = Georgia Tech
1989 - no undefeated teams = no national champion
1988 - one undefeated team = National Champion = Notre Dame
1987 - one undefeated team, one one-tie team = National Champion = Miami (Syracuse)
1986 - one undefeated team = National Champion = Penn State
1985 - no undefeated teams = no national champions