Since I started the Dark Brotherhood questline, my game has been crashing alot, after I fast travel to a location...

I running on PS3..

either you have a corrupted save or something is wrong with your game data.

I would suggest going into your PS3 system files and look for the Skyrim game data. Delete it all and reinstall it including up to the latest patch. Don't worry about your save data as that is in a separate area of your PS3 so it will still be there. If this doesn't fix your issue you may have a corrupted save file. Only thing I can suggest there is to revert back to an earlier save if you got one and see what happens.

i don't play on the PS3, but I believe this can be a symptom of save game bloating can't it?