On a lot of systems, Sonic Adventure 2 does not run in fullscreen mode, or it even has trouble running at all in windowed, or is resolution dependant on it working or not. Luckily, there is a mod loader which can help with this, as it has built in functions which change the graphics settings of the game in ways that the regular configuration program does not let you.

• There is an installation guide on the download page [gamebanana.com] of the mod loader which tells you how to set it all up and get it working
  
• Open SA2ModManager.exe inside of the Sonic Adventure 2 steam folder, go to the graphics tab, then enter the resolution of your monitor if needed (and if you have multiple monitors, make sure that the monitor you wish to use is selected)
  
• Select "windowed fullscreen" under the resolution section. This technically runs it in windowed still, but it essentially looks like fullscreen, and does not minimise or crash when you use another window on another monitor too!
  
• Tick "lock framerate" lower down in the options to lock the framerate. The reason why your game speeds up if you are using a monitor with a higher refresh rate of 60hz, is that your physics are tied to the framerate, making the game run faster based on the framerate. This will lock it to 60 FPS, making the game run at the speed that it should
  
• Click "save", and then go into your Sonic Adventure 2 folder, and rename "launcher.exe" to anything other than its original name, and then rename "sonic2app.exe" to "launcher.exe". This will make steam bypass the graphics settings launcher, which messes up the graphics settings controlled by SA2 Mod Manager.

Now you should be able to launch Sonic Adventure 2 in full screen without any speeding up issues or crashing!