I have the exact same configuration as yours but I do not own it on Steam, only the physical version. It also crashes for me rather randomly. I will try to reinstall Windows 8 and 8.1 this weekend.
I assume you have the latest patch and the drivers updated.
Do not forget to launch the game via LaunchIV.exe
I believe what should fix it is using a commandline:
Create a new text file named commandline.txt and place it in your GTAIV directory, and include:
-restrictApp 0 (don't sure if this one works)
OR try using what R* posted here. I know you already tried but here is a correct way to use it:
If you are experiencing frame http://bourgeonsdepl...-122906978.html fluctuations, please update to the latest video drivers, then use the -memrestrict commandline. Please do not use any other commandlines as they may override the effects of this one. Start at the lowest number based on your texture setting (ie. try them one at a time) You can try to increase the number until you notice stuttering again.
-memrestrict 188743680 LOW TEXTURES (256M Video Cards)
-memrestrict 262144000 MED TEXTURES (512M Video Cards)
-memrestrict 629145600 HIGH TEXTURES (1Gig+ Video Cards)
If you still notice stuttering with the lowest number, there may be a seperate issue with your system configuration.
Hope it helps -- that is what I am going to do this weekend. You can search for some commands as well as I reckon I do not know them all. Maybe some other members will help.
You can also use XLiveLess but it will remove the multiplayer (it is said that it will be shut down soon anyway) and you wouldn't be able to send videos I believe, but if this is your last resort, you should give it a try.
If you want more information: http://pcgamingwiki....d_Theft_Auto_IV
I truly hope it will help! Good luck!