Login trouble

Hi, i just installed a clean WordPress 2.0 on my server (Brinkster, its a shared hosting server). After about 1 week using it, i noticed that some times when i want to post something it ask me for my user and password and I lost all the information i justa wrote.

Note this happens happens randomly, sometimes when I’m writing posts, on changing themes files, on uploading files, etc…

I really want to know hoe to get rid of this becasue i’m tired of login at any time.

Help!!!