You might not want to use Replace cars as it makes the game non stable even if you have a modified gameconfig. You should always prefer add on cars.
If really you want to have replace cars for police vehicles in LSPDFR here is what you can do :
decrease the size of the textures of all your replaced cars to 512x512 max and it will solve your texture flickering issues.
for each car.
Open the ytd file using OpenIV.
On the left hand corner you will have the size of the textures (example : "4096x4096, DXT1, Mipmaps:9")
2)Extract in jpg format on your desktop each texture where you have a number above "512x512"
Open Photoshop, or any image editing software.
Load the Jpg texture. Go to options, Resize and reduce the size to 512x512 maximum. To compensate the loss of details, you can sharpen it a bit.
Save the modified texture in .jpg format on your desktop.
Reimport the modified texture in the ytd file with OpenIV.
The vanilla cars YTD and YFT files combined often weight less than 5MB, whereas some downloaded car can weight 20 to 30mo (some car creators put useless details such as 4096x4096 textures for Tail light or tires... if you want to compare,the size of a tail light for Vanilla vehicles is 256x256 pixels).
Your goal should be to reduce the car ytd size to 15MB maximum. Below it's even better but you will lose details.