Open your RPH settings, then go to the Plugins section, and don't load anything on startup. When you're in-game, try opening the console and ReloadAllPlugins. See if you can go on duty from there. Also, make sure your graphics drivers are all up to date.
Open Task Manager (shortcut Ctrl + Shift + Esc), go to Details and look for GTAV.exe. Right click it and set your priority as High or Realtime. Next, open your lspdfr.ini and set Main.PreloadAllModels = false. Now restart and retry your game.
Regards,
Zen