Random "access denied" problem

After 7 months of no problems, in the past couple of weeks, my forum has starting giving "access denied - you do not have suitable permission to view this page" errors when a person tries to view a topic. (happened on a wide scale twice in a week)  This seems to be randomly effecting anyone who is on the board, some continue to post ok, others get blocked.  Even us admins get the same error message, which is very annoying to say the least.

The strange thing is, you can post new messages, but then can't read them!  PM's continue to work ok and I can access the admin section ok too.  Sometimes it clears itself if you shut down the browser and reopen... but only sometimes.

Any ideas please?   My forum is pretty busy with hundreds using it at any one time, is there some sort of limit written into the code somewhere that restricts access if demands are too much on the database?  mind you, its been busier in the past and I've not had this annoying access denied error appear.

Thanks in advance.  Keith.

thanks for replying. On our hosting package, we've recently signed up for load balancing across a number of the host's servers, so I guess when its switching servers in the background, it could be resetting the sessions??  

The timing of us switching to this and the problem starting, certainly matches up.  Cheers.