cookie lose its value randomly

[Seth Broomer]

i have a site that stores about 5-6 cookies

randomly the cookies will either expire, or just lose its value

what i'm doing is pretty simple.

when the user logs in i save some of their data to a cookie

HttpContext.Current.Response.Cookies["username"].value =username;

on all my pages i get that cookie via the Request.Cookie object

but then randomly clicking around the cookie disapears. is it expiring?
i want it to only expire when the user closes the browser

thanks
seth