Does the coolant level keep dropping? If it does take a look at the spark plugs. If one or two look different than the others, it could be a sign that they're getting affected by coolant(cracked head/block/blown or leaking gasket). Could narrow down your search.
If the level stays consistant you might try drilling a small hole(1/8" or slightly smaller) in the thermostat body to let air out.
Like 64nailhead suggested, get a vacuum gauge on it and record the readings. Disconnect the IAC when its cold just after its started. Note the results. Let it get cool and reconnect. Start it and let it warm up, wait until it starts surging again then disconnect the IAC again and note the results. Be ready to shut it off if it starts to race or stall.
Good idea to check fuel pressure cold and hot as well just to rule it out.
Whats the temp. gauge doing?