I found MXM was too buggy so I switched to UnleashX. Almost the same features and very stable.
Freezing in the dash:
Are you using any preview videos or thumbnails? Make sure the XML is pointed to the right files; then reset the cache. Make sure the video has the right encoding. Did you tripple check your XML to make sure you have the correct Tags and end Tags? I also found MXM buggy when browsing for a game/app that didn't have a save folder yet; it would crash.
Freezing during game:
I'm assuming the game is being run from the HDD. How did you upload the game to the HDD? Some apps upload games and exclude some files to save space. Some apps just miss some files durring uploading, so use an uploading specific app since that's their main focus durring developement. I always upload my games with ACL turned off in DVD2Xbox.
1st try deleting the gamesave and cache and see if that fixed your problem.
2nd if that didn't work, then try reloading the ORIGINAL game disc to the HDD with DVD2Xbox using ACL disabled.
3rd if that didn't work then try the original game from the DVD drive.
I really recommend you to convert to something like UnleashX since MXM is no longer a supported dash. All the bugs will never be fixed until someone starts work on it again....but it seems unlikely.