I have an '87 5.0 LX with 44000 miles. After driving it for about 15 minutes the RPM will jump to 2500 RPM and stay there. After coming to a complete stop it is still at 2500 RPM. If I shut it off and restart it it will be fine. It seems to do it when the weather is warmer.
Please read the Useful Technical Thread Index sticky at the top of the forum. Inside that thread contains a link to the Surging Idle Checklist, which will likely contain a fix to your high idle issue.