Alright so I've been having these problems only in GTA IV v1.0.7.0 while my EFLC doesn't have this sort of problem (although I think it happened there once or twice too)

Sometimes when I initiate a traffic stop, they nicely indicate, pull over to the side, and then when they'd try to adjust their vehicles next to the curb they always fail and move back towards the traffic. From this moment 2 things can happen:

1 is that they turn the engine off and then you can get out and continue your traffic stop (usually without crashes)

2 is they turn their engine off but then immediately back on again, and then they don't pull over like you were supposed to be in a chase, however this is not the case. The traffic stop mode for some reason after that restarts or I dunno, because I can pull them over once again, and the same shit starts happening all over again. Once it goes bad, only the 2nd type of thing will happen. While they are in the traffic stop mode, they drive nicely in traffic obey traffic laws, but don't pull over for your siren. Even when you get in front of them, you get the 2 markers to "continue" with your traffic stop, but they'd obviously drive away while you are stuck in the traffic stop screen. When you do eventually get them out, you can order them to stop (sometimes results in crash), they stop for a second, then start walking away, if you try to arrest them, they put their hands up, like they are ready to be arrested, but they once again, start walking away (and then the game either crashes or not). While they are walking away you can get behind them and your character will slowly start walking towards the ped, but since they walk away (or sometimes even run), it's almost impossible to cuff them. From this moment my character walks on it's own, and I can't stop it.

I never get any error messages unfortunately, but this last time I was playing I had the same bugged traffic stop, but the ped started shooting at me, and that's when I crashed. Finally I got an error I code I can report, I'm not sure if this is the same problem error I get, since the other times, I never get any error logs.

[ERROR - 5:43:38 PM] [Main] CRITICAL ERROR DURING MAINLOOP! REPORT THIS ISSUE AT LCPDFR.COM BY INCLUDING THIS LOGFILE.
[ERROR - 5:43:38 PM] [] System.NullReferenceException: Object reference not set to an instance of an object.
   at freval_78.eval_fr10(Object[] A_0)
   at LCPD_First_Response.Engine.Timers.DelayedCaller.fr0()
   at LCPD_First_Response.Engine.Main.fr2()

Can you please help me out, because this is really irritating, and about 50% of the time this happens.

The freval error was fixed for some users by doing this: Open up your LCPDFR.ini, look under "Main", it should look something like this by default: 

[Main]
AdditionalCopCarModels= 
SuspectTransporterModels=POLICE;POLICE2;PSTOCKADE; 
VehicleTrunkWeapons=Shotgun_Baretta; 
AllTextMode=False 
AmbientScenariosMaximum=2 
AmbientScenariosMultiplier=250 
DepartmentName=Liberty City Police Department
DepartmentShortName=LCPD
FirstStart=True
UsingPoliceStationMod=False

Now, add to the bottom of that part

PreLoadAllModels=True

The "Main" part should now look like this:

[Main]
AdditionalCopCarModels= 
SuspectTransporterModels=POLICE;POLICE2;PSTOCKADE; 
VehicleTrunkWeapons=Shotgun_Baretta; 
AllTextMode=False 
AmbientScenariosMaximum=2 
AmbientScenariosMultiplier=250 
DepartmentName=Liberty City Police Department
DepartmentShortName=LCPD
FirstStart=True
UsingPoliceStationMod=False
PreLoadAllModels=True

Thank you for your quick reply, I'll try it.

Let me know how it goes :)

Nope, didn't work. Still having the same problems. Also if the suspect trys to escape for real, then even though you can report the chase, but every time you have the siren on, he pulls over like a normal ped.

I'd say that is another bug to add to the list. I'm not really sure what could be wrong. I did try to reproduce it, but I didn't experience the same issues. 

I'm not sure, but this problem might be related to my GTA IV, but this is weird, because I haven't done anything to it, it just started doing this one day, and it hasn't stopped since. Like I said in EFLC 99% of the time it works. They even pull over properly, but here they can't even pull over properly, I mean it's like, pull over, start going back in traffic, turn off engine, so they end up on the road lol

Well, sadly that's GTA IV for you. You know how it is... haha. The only thing I could recommend is reinstalling, apply latest patch and install only LCPDFR 1.0a with the manual installer. Kind of generic advice sadly. 

Yeah I think I can confirm now that this bug is related to my GTA IV only. I don't think a reinstall will fix this, however I did find a kind of solution.

This only happens in multiplayer I might add that.

First of all I tested it in EFLC, SP and MP, works flawlessly, no problem at all. Then when I go to GTA IV, it works properly in SP, however every MP game I join, even if I host my own server this bug occurs. The "fix" seems to be that, I have to disable then enable the traffic and walking peds with the simple trainer (options, 2nd page, enable/disable cars and enable/disable peds), after that I put reloadscripts into the console, and voila, lcpdfr works just as it should be. I'm running pretty much the same mods in both of my games, except I got a CA pack in EFLc, and NYPD in IV, so I really don't understand what might be causing it. Anyway if anyone has a similar problem, try this method.

Huh, that is really interesting. Weird, but interesting. Thanks for letting us know.