Why am I not getting power?

I'm having this issue with my old 88 c1500 Silverado, it has the 305, it's not getting any power acting like the battery is dead. But every once in a while I'll put the key in and get power then flip the key to start and nothing happens. Then I flip the key back and try to start and get no power at all, nothing will power up. I did replace the battery hoping that would solve my problem.