I think some of that has to do with the ear issue she was dealing with at the time in mid-2007. I would say that her TV lives have always been rather "okayish" but 2007-2008 was probably a dark time for her vocals especially. Since that period of time though, I think she's been improving steadily except for the hiccup she had with You were... and evolution which she acknowledged herself. I'd still like to know wtf went on with that performance.
Just for some performance comparisons...
HANABI from 2002
No way to say from 2003
JEWEL from 2006
Love song from 2010
Life from 2011
http://www.dailymotion.com/video/xht3po_life_music
I think at some point between 2003-2005, she stopped singing with that nasally voice that she developed at some point after 2001... and somehow that was her 'comfort zone' when it came to singing live? I wonder a lot about her vocal choices sometimes when I watch older performances like... why did she decide to start adding a-a-a-a-a-a--a-a-a to everything and how come she stopped using that nasal voice?
Sometimes it seems like she might not be warming up properly for TV lives so it sounds like she isn't getting enough breath in when she's performing. She doesn't really have this problem during her own concerts so it's a bit strange to me.