You said "pc" so were you talking about microsoft windows? I'm a little
confused by the terminology, but since you're posting here I'll assume
you are running ubuntu.

My first thought is that it's probably not graphics specific; rather the
slow graphics is just a symptom. I suspect another program might have
gone astray, and disappeared from the screen but is still thrashing away
and hogging the CPU - something I've seen with browsers and other programs.

Open a terminal and run "top" and based on the output, consider these
questions:

What is the load average on the machine?
Are there any processes near the top of the list, using a lot of CPU?
Is there any swap being used?