I figured it out!! After a full day of troubleshooting, I concluded that it has something to do with the latest version service pack 1C. I had an older version of PMDG 737 NGX on another machine that worked flawlessly, and when I loaded it on my new computer (homebuilt Intel i5 3.4Ghz, 8GB ram, AMD R9 270X,128GB SSD, Windows 8.1) I never saw any issues again. If you are having issues, try to find the base version 12.0.0. Only downside to the base version is your frame rate will go down a little bit, but hey, it's better that letting it crash your session. Tested wih REX Essentials + OD, and ORBX.