Should show 12.6 volts.....whats the voltage when it won't crank? Do you have a bat gage in the dash? What's it charging at?
Look at your THEFT/SECURITY light when it won't crank(release key when it won't crank and leave on)....if the light is on, that's your problem.....by the way, you don't have one of those aftermarket remote starters or theft systems added to your vehicle do you?