flashing lights

I have a 95 blazer 4x4 automatic with a 4.3 vortec in it. my problemi is a few things the first is when i start it up let it idle or driving it to. All of the lights start dimmin than high like a cop all the dashboard lights do it everything does it the volt meter goes up an down with it. And when i put a battery in it and put the neg. cable on then the pos. it starts right up with no keys even in the ignition an then when i get in to turn it off the keys won't work the only thing that turned it off was pressing the brakes. so if anyone can pls help me out i have bin trying to figure out what its bin for a long time!!!!:disappoin:disappoin :banghead::banghead: