After about the 3rd play through most if not all NPC have stopped moving and just stand around.

Vault 3 the Fiends will not shot back.
The ants at the dry lake/race track just stand in one place and never move.
A lot of the gouls (well most of them) act like the ants do until you get right next to them.

The first play through all the Rangers at the Rangers Stations move about and have set path but at the 3rd play through they will only do the path once and then don't move anymore.

Yes I know I can uninstall and reinstall to make it work the way it does on the first play through but that will take a lot of time to do.

So how do I reset NV to play like it does on the first play through without having to uninstall and reinstall.

Are you using any mods.

Is each playthrough with a new character.

Also it would help us if you post a full dxdiag.exe report in spoilers tag.