If it only happens when you are re-entering areas which you have already visited, then it might mean there is corruption in your minisaves. Let's hope this is not the case; it raises unpleasant questions.The simplest possibility is that you don't have enough *free* memory to support the work the game is doing when it tries to setup the level. Close all programs running in the background, especially any firewall and anti-virus programs. Of course, you'll want to disconnect from the internet if you have an 'always-on' connection.
