We need more information to help diagnose the issue(s) you are having:
The specification of your computer system including Windows version and program version, see this topic for what is required and please quote processor and graphics card make/model in full, and also what monitor/screen resolution(s) you are working with if this is a laptop.
I would suggest you put this information in your profile signature so we do not have to keep asking for it.
What is the full error message, a screen shot may help?
Are the graphics card/integrated GPU drivers up to date?
A screen shot of the Program settings, Device options tab may help.
It the issue occurred when adding video to the timeline what video format, codec and resolution are you using - to help with this download and install MediaInfo and analyse one of the clips causing the issue and post the results, see this tutorial on how to setup MediaInfo and analyse a video clip for all the data required.