I put a k+n open filter system on my 4.3 vortec and have had problems with starting and basic performance. I put a jet chip in it and it seems worse. It is only hard to start cold. No leaks in the fuel system. It almost sounds like it's missing so I replaced all the ususal stuff plugs wires filters excet. no luck. Could high flow be incompatable with the mass air sensor?

 

I'm betting that you have a dirty MAF sensor. Run a search and you should find a ton of information on it. Here's a quick reference:

https://blazerforum.com/fb.asp?m=26733

Good Luck