Video Playback & Import Works Intermittently - HELP PLEASE!

Hello, I am having an issue where my sessions will work just fine with video, able to playback and import video with no issues and then sometimes I will open the same sessions that had the video imported to them and the thumbnails are gone and the video will no longer playback. When this happens, also with new or existing sessions, I can no longer import video files that I had been able to import with no problems previously. This is causing me much grief as I am doing lots of sound design to video for my job and am losing major time and productivity. Please, can anyone help?

Yes, I have the latest version of Quicktime installed.

Here are my specs:
Cubase 6.5.4 Build 161 (64 bit)
Intel® Xeon® CPU X5560 @ 2.80 GHz 2.79 GHz
RAM 8.00 GB
64 Bit Operating System
Window 7 Professional w/ Service Pack 1
Display Adapters:
NVIDIA GeForce GTX 560 Ti (Two monitors)
NVIDIA Quadro NVS 290 (Third Monitor, HD TV w/ PC Input)