I looked over the surging idle checklist and ran a KOEO test, the car flashed 1-1 for a system pass. I had a hanging idle at ~1500 rpm that was intermittent the last 2 or 3 days. I drove the car yesterday and it seemed to go away, the car idled at lights where it normally does around 950 rpm. I looked for loose hoses under the hood and everything seemed fine, although the plastic that holds the "salt and pepper shakers" seemed like it wasn't connected to anything, just sitting behind the intake. I have a GT40 on the car so does this not have a spot for the plate to slide into or do they just kind of sit behind the intake like that?
The salt & pepper shakers can float if you don't find a place to latch them down.
The intermittent nature of your problem suggests that it is time to clean the IAC and throttle body. Then do the base idle setting proceedure at the end of the surging idle checklist.