I would simply say that your Library is corrupt for some reason. As your library is a virtual reference lookup to your actual music files, if it has a problem it is possible that it will either give you issues at program startup or perhaps when you add something to your library as it will read the reference file and encounter issues when trying to add to it.
I first of all tried all the obvious, Disk Util repair, permissions, deleted and reloaded Itunes etc etc however all of that will still use the original Library file so it wont solve the issues so many are having with Itunes.
The simplest yet time related route is to rebuild your library. What this means is that you need to create a new one, and import all your content again.
You may be thinking - oh my god, forget that! however its not that bad!
It will take you 30min but your computer 10 hours.
As its your library thats corrupt not the actual files / file structure that is the problem you will be able to import all of your music again together... 95% of your ablums will be kept together although in some ocassions with downloaded albums their reference name will not match that of the official release so it may be seperated and in need of the artwork but neither is a big take to sort out.
Its the same deal with your movies.
So to do this, quit ITunes... then hold Alt and click Itunes again, select new Library (dont worry your old one is still there and will not be deleted).
They you can start to rebuild. You can always go back to your old Library by closing Itunes, holding Alt and clicking on Itunes.
To select which library to open - you will see that in your Music/Itunes folder there are now two libraries and the xml library file is in the sub folder for each for you to select if you cant find it... give yours a simple name when you create it such as 'ITunes Libray 2' that way you can search for it too if you cant find it.
The only issue with this method is having to backup your Iphone / Ipad or other device in your old Library as it can only be sync'd with one library... however once you are happy with your new library you can Erase your device and sync it with your new Library.
Fear not, when you erase your Iphone for example; it will only wipe / erase the content (Music, Movies etc) it will not delete Apps, Messages, Bookmarks, Photos etc.
I hope this helps, as it not only sorted my Itunes issues for good but also made my IMac a hell of a lot faster in general as Itunes is not struggling and does not crash any more.