Login problems

Hell, i need help and i need it quickly
Every 3 or 4 days i have the following problem:
users login fine, but when they open a topic or a post to read it says you are not logged in!!!
Same goes for me as administrator..

I already had to reinstall it twice, backing up my database and importing it back in. But that is not the solution, erasing installation and going trhough it all over again...

I like this forum but if the problem persist i will have to move to some other

Can you please help?
Thanks!