Try reinstalling Roblox. If that doesn't work, please start Command Prompt, then type "SYSTEMINFO" and then after it's done loading, post the OS Name.
[or just tell me what opperating system you use]
It could be your computer. My computer does this to me. But my desktop make it normal. I tihnk you could also try getting a new graphic card? Or maybe downloading service pack 2. -Spark