What exact version of Pro Tools are you running?
What version of driver for your interface? And importantly, confirm you are not running ASIO4ALL.
Lots of good advice sofar. You are unfortunately lost on some stuff here and you do need to get things you are doing right.
You also do not mention the most basic very relevant info like IO buffer size or sample rate you are running Pro Tools at, or if you have ignore errors checked... if so uncheck that, you want to see any AAE error. What is disk cache set to? with small test sessions just set that to 1GB or so and let the session fully load (disk cache meter turns green and has significant free space) before starting to do anything.
Looking at meters and % CPU rarely tells you anything useful and is likely a waste of time. That's been discussed to death on DUC before.
Since the 6i8 has optical digital input...are you using that for anything? is that input set to be the clock source? if you are not using it disconnect anything connected there and make sure the clock source is set to internal in the 6i8.
There are standard troubleshooting directions under "help us help you" up the top of every DUC web page. starting with trashing prefs--always start there, and especially trouble shooting plugins by moving aaxplugin files out of the plugin folder (and I would trash prefs again after doing that)--always suspect plugins, yes even ones installed but not being used. There is lots of standard troubleshooting stuff you don't seem to have tried... or if you have you need to tell us.
Purchasing the PT Trasher app will make trashing stuff easier.
http://www.pro-tools-pc.us/category-s/1514.htm
Still stuck after working through all this describe all that standard troubleshooting you have done and what you saw (take notes as you go), and post a SiSoft Sandra report.