403 - Forbidden: Access is denied" Randomly comes up when editing site???

Would love any help for the following please:

What seems like purely random timing when editing the site the 403 page appears - "403 - Forbidden: Access is denied. You do not have permission to view this directory or page using the credentials that you supplied" then I have to delete all site cookies then re login and it goes back to normal.

Sometimes I can be editing for 5 mins and it will happen other times it might not happen all day.

Really don't want this to happen to customers when there using the site as most will then just think it's a broken site.

regards

Did anybody manage to solve this? I would be very embarrassing when the customers see this when they visit the site. By the way, I am using version 4.50.2.
Thanks,

Check the System Log for a more detailed error message