There could be many reasons why your simulator could be freezing. Here are some reasons why this might happen:
1. Addon aircraft might be saving their own data and causing freezing themselves. Freezing while using PMDG products is a known issue.
(We recommend using a higher save interval while flying these addon aircraft that cause this issue)
2. Windows file caching. If Windows is not set to cache files it will try to save the files directly to the hard-drive and causing some freezing while its doing so. (We recommend setting Windows to cache files.)