Engine revs up when starting

[johnyoung]

Fellers I have a real problem on my hands. 09 - 5.3 out of a 2 wheel drive chevy , brand new motor and trans with 900 miles on it. Install went great in my 67 SB pickup. Problem is when I start it up it revs to about 3000 rpm's then slowly comes back to idle. It even does it when I stop and put it in park and rev her up a little - I called the guy that does my harness's and computer and he has never heard of this. Replaced the throttle body twice - no change. Any ideas will be greatly appreciated.