Hey! I also experience random crashes both in single and multiplayer modes. If it happens, it's usually just at the end of a loading screen.
It might be a bit drastic, but when the game starts messing up too frequently, I sometimes just reinstall everything from scratch.
Backup your saves (if you have any) in AppData/Local
Uninstall the game completely + delete the entire GTA IV installation folder
Uninstall GFWL components (LIVE Redistributable + Marketplace)
Uninstall (if applicable) Rockstar Games Launcher + Social Club (if installed)
Be careful not to tick the boxes to avoid losing your saves in games with the Rockstar Games Launcher (if you have any, of course)
Once the game and all the above components are properly uninstalled (including the complete removal of the GTA IV directory), do restart your PC
Then you're ready to redownload the game and start the downgrade procedure.
Personally, I followed this tutorial which works pretty well: https://steamcommunity.com/sharedfiles/filedetails/?id=2827422884
The game runs relatively well in both solo and multiplayer for me.
You can also try enabling compatibility mode for older versions of Windows, like Windows 7.
The game is quite notorious for being unstable on many systems. Sometimes patience is required, what works for others might not necessarily work for you. You'll find a multitude of tutorials online to try and improve game stability.