I just replaced my intake manifold on a 2001 f150 5.4 and now I'm having an idle issue I did check all electrical connections and vacuum hoses/lines even replaced the IAT sensor but when I start the truck up the rmp's run up high and then drop down almost dying and some times dose I am at a loss for what the problem is need help please

 

Cold start sequence:
Key on samples the outputs of the CHT and IAT.
This tells the computer what operating conditions to set up for.
Normally higher idle is from the IAC opened up for more air.
As the temperature of the CHT rises and the OX sensors come up to temperature the idle begins to drop.
.
Since your rpm goes high that tells you the IAC has been opened up.
Dropping too low past normal idle control would indicate the IAC has closed up cutting air off.
Or there is an air leak allowing excess air in causing the computer to shut the IAC down and excess fuel injected making it to rich.
.
I would first suspect the IAC of a blocked air bled inside its housing.
This can cause a crank/start then almost immediate stall.
Good luck.