rkcbvw

Pulling my hair out on this. Since initial swap this 5.3l started hard but after 2-3 seconds of cranking it would finally fire and then ran perfect. I could shut off try to start again and same problem. Only way to get it to fire up immediately was to spray starter fluid in intake. I am noticing it get worse now as last few nights I cant start it at all unless I shoot starter fluid in the intake. Without starter fluid it cranks and cranks with occasional fire but does not take off. Plugs are brand new and I am getting spark as well as fuel pressure is good (60psi). Also I noticed now when I run it and step on gas it makes a gulping sound like it is gulping air and it stumbles which it did not do at first. Thinking I had intake leak I sprayed starter fluid around intake while running and no change in rpm. Seems like I am missing something simple. Any advice would be much appreciated.

so far it is stock motor with 120k miles dbw.