If you engine starts and runs fine for say 15 minutes or so then I would think you do not have air in the lines.
Another thought you could have an air leak, check all you connections.
On my 2GM it takes a lot longer than 15 minutes at 1000 to 2000 RPM to run air through the system. Closer to 30 to 40 minutes. Hard to believe, but I learned the hard way!