Just a guess, but could this be the multi-core problem that plagues fallout 3?
Open fallout.ini (Documents/My Games/FalloutNV) and paste "iNumHWThreads=2" (without quotes) at the end of the General section.
Not sure if this will work or not for NV/TTW, but this does fix the random freezing issues for F3.
If it still freezes after trying that ini edit, try starting a new game without any mods, and see if the freezing still happens. If the freezing stops happening, then it's related to a mod you have installed.
