Usually it's a leak in the vacuum system. Check to see how the brake booster tube is fitting....that's a lot of air. The other spot to check is the gasket below the EGR valve.
To clarify a bit: The intake manifold needs an airtight fit everywhere, or it sucks in more air and the vacuum drops....increasing the idle in the throttle body. You need to seal up any little cracks or holes that let more air into the manifold....cracked or missing vacuum tubes will let a bit of air in....but the rpms will not go up to 2500.....you need more air to get that high..
[This message has been edited by Gall757 (edited 01-29-2015).]