Can you confirm if you're ticking "Remember Me?" when you login?
If not, it could be that your login cookie is timing out - so you're effectively not logged-in when you try to reply / post.
Or ... having re-read your reply, are you saying that you try to log-in, but the username field resets and you have to make multiple attempts at logging-in before it finally takes?
Cheers,
Shaun