Starting problems

I have a 2004 mitsubishi endeavor which has a starting issue every once in a while. It also has a no power to the doorlocks and radio. There is no blown fuse yet still no power to either. I have a friend with an eclipse who had a similiar issue and said he found it to be a cluster relay was bad. If anyone has an idea as to what to check and where?

Have you checked the battery cables to be sure they are clean and tight, as well as checking the battery voltage on the battery posts when it won't start? It's not unheard of for a post to break inside the battery case.

-Rod