iPhoto freezes when I try to change the tags (names) to faces

I have a large library in iPhoto and I have over 1,200 pictures of my daughter that I have tagged as her, however about 50 of these are incorrectly tagged. No problem (I thought) I will correct these, initially I tried this by right clicking in the faces tab on the picture in question and chose the option that said ' This is not XX' - but iPhoto crashed, I then tried the more normal way of doing this by selecting the option that is for identifying her in more photos, initially I changed the ones back that wanted and then preceded to select and deselect the images that iPhoto offered me.

This has now crashed 4 times and when I get back into iPhoto things are found to be as they were before I started.

I need some help with this. All apps are up to date. The library itself holds 34K photos at present and the hard drive is 3/4 full The iMac I am using is about 12 months old and running well.