HEI firing question and problem...

[Novaguy73]

I have a 72 chevy C-10 i just put a new HEI in. I put the 12 volt source for the coil pack on the "radio" spot in the fuse box. It didnt fire. I then switched it to a constant 12v source and it fired right up. I then switched it back to the radio spot and IT FIRED right back up. Then i let it sit for a few minutes and it wouldnt fire again!! Swith it back to the constant 12v source and it fires right off and then back to the radio spot and it fires again, then after 5 minutes or so it wont fore again. my only problem with it being on the constant source is the it wont shut off...thats a problem. The good side is that it runs like a CHAMP. Im talking it cleaned up the idle and has great throttle response and acceleration compared to the old points dist. Could it be a bad battery? i dont know what else it could be... any ideas?