CarlSkotch

I personally use this one: 
https://www.gta5-mods.com/scripts/fix-wheel-turn-back 
Works like a charm. You need to create a 'scripts' folder in your game directory (if you don't have one). Hope this helps.

1) In the vehicles.meta, make sure the problem add-on cars have FLAG_LAW_ENFORCEMENT and FLAG_EMERGENCY_SERVICE
 
2) Make sure these vehicles all have a carvariations line that includes sirensetting=1

Thank for clearing this one up. However, how does it refer to Computer +? I had all the flags in meta files so carvariations line fixed that issue too. 

I am experiencing an issue when I play LSPDFR that after some time (can be from 5 to even 45 mins) I have frame drop from exactly 60 fps to 20. It is always the same and the game stays at 20 until I restart it. It usually happens when I call for backup, accept callout, arrive on scene, or even wheather conditions change. It is pretty random. When I run my well modded LSPDFR (emergency cars installed, many plugins and VisualV) CPU is at about 65-75% and RAM is 60-70%. 
My specs are more than enough to run the game and I have applied decent video settings optimized for my GPU. Windows and Nvidia drivers are up to date. 
Just in case, my specs are below:
i5-4690K (not OC) 
GeForce GTX 970 (not OC) 
8 GB Ram (DDR3)
Windows 8.1 and both system and GTA 5 are on SSD 
I have tried many solutions and neither of it helped me. It pretty much kills my gameplay so any tip would be much appreciated. In addition, it never happens in vanilla GTA 5/GTA Online. 

Some police slots are not multi-livery enabled. To enable the liveries, go in vehicles.meta in update/update.rpf/common/data/level/gta5, extract it, open it with notepad or similar software, find the lines of the slot you placed the car in and scroll down to a line with a series of 'FLAG_HAS_STUFF'. Add FLAG_HAS_LIVERY at the end of it, save, replace the original vehicles.meta file with the one you modified, done.