what would normaly cause a backfire thru the intake when you first hit the gas? doesnt do it all the time but on a regular. it cranks fine and idles fine but once things warm up sometimes it has problems. it mostly does it during cam surge

 

Have you changed the plug wires lately? That can happen when you cross plug wires.

 

Check them all please easiest way to diagnose prob'

 

Other thing to do would be to unplug the O2's and drive it around for a bit.