Greetings,
I was head banging all day today trying to load and run a 4.5 million line Gcode.
It would load then lock up.
Had to zero my machine four times today on all four axis as a result.  What a p-sser.
So I PM'd Brett and he got back to me in no time with the solution.
Go to Diagnostics>Toolpath on/off and click to Off. 
This can be helpful IF the computer used to run your miill does not have a graphics card.
MOBO's native ability to deal with the display of tool paths can cause problems.
Thanks to Brett and the great support I have gotten on this Forum. 
Wish I knew enough to contribute, but I'm just a bottom feeder.
ciao,
Larry

Thanks Larry, glad we could help. I may be the only one you talked to on this problem, but I have talked to thousands that have helped me to help you. Great group here.

Brett