Texture packs are just textures: the advantage is they make stuff look different, the disadvantage is they generally use up more RAM. If they use up too much RAM, that can crash New Vegas if you exhaust the 32-bit address space (2 GB by default, 3 GB or 4 GB if you have a large address aware patch for New Vegas).
The stuttering would be because your computer is choking due to the extra long time spent getting an extra large texture from your hard drive into RAM. Defragmenting could potentially help some, but ultimately you either need a faster hard drive or smaller texture files. It could also be from increased page file usage.
Freezing, if you mean a hard freeze where you have to restart your computer, is a little more bad and really would indicate hardware trouble... at worst the game should just crash. If you just mean the game freezes and not the entire computer, eh, could just be New Vegas being New Vegas.
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