Why does my iphone sometimes connect to Itunes and sometimes can't?

Sometimes i can connect my iphone to my computer and it says iphone apple device. But sometimes it says device unknown? Can anyone help on this? Also, my Itunes need to open so my iphone can connect. I don't know why.

One of the things i would cheak first is make sure both iTunes and your iPhone are up-to-date. If not cheak this out.
http://support.apple.com/.../kb/TS1538