Replaced peds don't work after a while

[Omina]

So I have a mod which replaces the ped skin ig_anthony - when I install this with OpenIV and replace the exisitng model, it works fine right after I install the mod.

 

But when I restart my PC and then launch the game again, when I try to change the skin from Niko to ig_anthony with a trainer, the trainer just freezes and the model doesn't load. Additionally, my last save will fail to load automatically so it will try to start a new game, but I can then load it manually. What's causing this? I have to keep reinstalling the mod to use it even though I already replaced the files...