PO300 code
 

I have a 2006 GMC Sierra, and i had the PO300 code pop up when my check engine light came on. I originally cleared the code, and after i changed my plugs it never came back.
Now my moms 2000 GMC Sierra is reading the same code. I know that the code is "random cylinder misfire detected" so i was going to start with changing the plugs on that truck as well. It is running really rough now, and last night i pulled 2 of the plugs, which do have a little build up on them, and the gap is way to big. I plan on changing them to the ACDelco Iridiums since that is the factory plug. Any other suggestions as to what causes this code by itself?