I have been running PT8 in a commercial studio for a month now with zero issues. Anytime I update anything, I always perform the update on a clone of my previous working system drive(so I can revert in minutes and get work done). It is a safety strategy I highly recommend(considering how cheap hard drives are these days, I keep 3, 1 PT8 load, 1 PT7.4 load[last working version] and 1 with the 7.4 load from 6 months ago). See this
http://duc.digidesign.com/showthread.php?t=193637
and this
http://duc.digidesign.com/showthread.php?t=133039
for starters. Then analyze what might have changed(besides the PT8 install). First thing that comes to mind is; do you have any 3rd party plugins? If so, they may need to be updated to work with PT8(drag the 2 files for each out of your plugins folder to the unused plugins folder for testing this theory). Waves plugins are notorious for this. Also, if you updated Windows, that may have reset some system settings and you may need to check all the tweaks(as described in the Getting Started Guide) to verify they are still set correctly for PT.