I keep getting bumped off

After a few minutes, I have to sign in again, getting sick of it. I think it has something to do with cookies, how do I fix it?

TA in advance
I tick the 'atuomatically log me in on this computer' box - then I don't get logged out.

Of course this means that my DH may sometimes 'visit' with my login name, rather then his own - but I don't think he post's under my name, just reads the threads.