Idle popping

My engine blew last year and I bought a 95chevy 350, I was running an original 89 350 before it blew. We replaced the engine, reattached everything and we got it to turn over and idle but every 1 second almost perfectly it makes a loud pop or snap sound. The idle is off, it will idle fine for a few seconds then die down like its going to stall and pick back up, we let it run for say 10 minutes like this.

Anyway we have checked the fire order, replaced the plugs and distributer. We can't figure out what the issue may be. Thanks.