MoreTextures

Are you sure you're installing them in GTA V\Plugins\LSPDFR and not GTA V\Plugins\?

Do you have any motorcycle/bike mods installed? These are pretty notorious for causing GTA V to throw a memory error.

1 & 13 is used by police.

As Starmix has already mentioned, 2 cards does not mean twice the VRAM, please don't buy a new GPU for this reason, you will waste your money.

While it would be nice, we can't actually edit the ped models themselves yet sadly, just the textures.

While it isn't at max (About 65 Celsius for a 6300 I believe?), that is awfully high for what appears to be idle temps. You may want to check your tower, fans, GPU, and CPU heatsink for dust followed by disposing of it with compressed air if needed (Just in case you aren't aware, please do not use a vacuum not designed for contact with electronics, this can cause damage). If dust is not a factor you may need to remove your heatsink, clean the thermal paste off, then reapply the paste and then the heatsink, or buy a new cooler entirely (I would not try this solution until it's confirmed that temperatures are actually causing your shutdowns to avoid wasting money or time). Keep in mind that while FX CPUs are fairly notorious for inaccurate temp readings (I know mine was), because your motherboard is reporting the same temp (Or higher in this case) for your CPU this does not appear to be a false reading. If you were able to get a reading of your temps while playing GTA V it may shed some light on the situation, if you are hitting above 65C directly before a shutdown it could be related. You may also want to consult a forum such as Reddit or something like Tom's Hardware if you haven't already, as you'll likely find more knowledgeable help compared to what I can do.

I'd download HWmonitor and/or HWInfo just to rule out abnormal temps being the cause here.

Just to avoid confusion here, dragging and dropping a carvariations.meta in will also overwrite it, you need to copy the text from one to the other manually. All though it was likely just a typo so no worries. Good luck modding!

Are you using an OpenIV installer/dragging and dropping the new mod's carvariations.meta? Because it sounds like it's being overwritten. You'll have to re-add the van entry in that case, likely 13 as usual. This definitely isn't normal .meta activity. Perhaps try a new dlc.rpf all together for certain vehicles if needed, I generally keep default car slot carvariations entries in mpluxe2, and additions in mpchristmas2 with their model and vehicles.meta entry (If that isn't what you're already doing that is).

That's extremely strange, I've never heard of that occuring. No, they do not have individual sirenSetting values, I have at least 10 with value 13 alone in mpchristmas2. Perhaps try adding a carvariations entry for the firetruck (firetruk) with a sirenSettings value of 7 (Note that is assuming your firetruck isn't modded, otherwise the value may be different obviously) in mpluxe2 or mpluxe (Or perhaps mpchristmas2 in this case...?).

I believe it might be 0x88000000, but it's been a while.

You likely need to add a carvariations.meta entry for police2 in mpluxe2's dlc.rpf. This is what you'd add specifically: This is most likely correct but it depends on the model, as it's hard to say with a Federal Signal lightbar as that's pretty generic, as it could also be sirenSettings 2 or 13 among others. If that doesn't work, you may want to link the vehicle mod you're trying to install and/or a screenshot of the issue.