When I turn the key on I have no power, it builds and starts. What do I check.

What is your question?

When I first turn on the key on my 1994 Mazda B2300 I have no power. The charge slowly builds to virtually full in about 30 seconds. Once running the voltage meter bounces considerably, but the truck runs without any problems( Until I shut it of and all power is lost immediately.) Do you know where I should start in checking things out?