Uninstall NVIDIA PhysX, run a registry cleaner, and then let the installer run
USE AT YOUR OWN RISK: Here is a .reg file that will do what the above is meant to do. After importing it into regedit, it manually sets the flag that Steam checks for during the installation process to know if it should install physx. The registry can be very delicate, and you should only modify it if you are comfortable doing so. I would strongly discourage you from using this unless you have an understanding of what the registry is and how regedit works. Here[pastebin.com] is a copy of the contents of the file.
Delete the installscript.vdf file in Payday's install directory. Note that verifying your install files will replace the file and it will need deleted again.
Assuming the problem isn't a problem with your network, you need to change your Steam download server location. To do this, open steam, open settings, select the downloads + cloud tab, and select a new server from under the download region section. The server needs to be DIFFERENT, not closer. Restart Steam and then try connecting to someone again. You may need to try multiple servers.
If that doesn't work, then contact Overkill Software (the developers) at firstname.lastname@example.org. If they give you a fix then please let me know what it is. ______________________________________________________________________
Go to appdata\local\payday and open renderer_settings.xml. UInder renderer_config, change the refresh_rate line to "refresh_rate = "60"" without the outer quotes. Use 120 in the unlikely event that it's supported.
If you can't find appdata, open the run prompt (windows key + r) and paste this: "%userprofile%\AppData\Local\Payday\renderer_settings.xml"
If that doesn't work
Download and install ATI Tray Tools.
Open ATI Tray Tools and go to Refresh Lock > Customize.
Select your resolution and match the refresh rate to your monitor's refresh rate. This can be found by right-clicking your desktop, locating your display properties and selecting the Monitor tab. Add the new entry to the setting list and click Enable Refresh Lock.