Problem with login to my forum its hit and miss.

[Realm Films]

my forum is at http://www.xlcinema.com and sometimes it takes two or three times to login, each time you login it looks as though its working "welcome user thanks for logining in, redirect yada yada" when it sends the user back to the site the user isn't logged it, but it says the user is logged in under the "who's on line" but it doesnt seem to work.

please try it, login:cervelo password:cervelo

if any one knows how to fix this problem I would really be thankful, I think its a database problem, but not sure how to tell, or how to fix it.

Shane.