Same problem as Stealthmaster and Wotan had time ago. Yes, the fat Wotan brought this desease...
Using windows xp, browser - mozilla firefox. Tried that delete cookies thing, ctrl + f5. ect. It started and spiked this week during the evenings. During the day everything works fine. But around 7 p.m. (+2 GMT) i get random log off. Username and password are accepted without problem. But few second laters when i press a topic/forum/top banner/refresh page - i get logged out. I come back after few hours, log in and the problem is gone.