I posted a few weeks ago about a troubled 2000 f150 I purchased. After replacing the air intake tube, and doing a full tune up (plugs and coils) the truck is back to full power. Apparently the trick not only had the wrong plugs installed by the previous owner but at least one of the coils were bad as well. Needless to say she is now firing on all 8... Now my problem is I have the horn sound in the intake tract at idle, also the truck will seddle into a 700 rpm idle, but as soon as you give it gas it will go up to about 1200-1500 rpm and hold there for a few seconds. It does this even when in gear and it's pulls. Makes stopping hard sometimes. At "idle" it can easily hit 30-35 mph !!!

I don't know if these two issues are related, but where should I start.

 

Change your iac first. If that doesn't do it look at your throttle position sensor. Mine just did this same thing not very long ago.

If neither of those check your fuel trim and see if you are running lean.