Here's my opinion:
The IAC is very finicky and even then, can just stop working for no reason. If there are pockets of air, or low coolant, it will start to hassle you.
Did you top the coolant back off when you were done, and burp the coolant? I mean REALLY burp it.
Like.. really, really burp it.
Other than that, make sure the 3 sensor connectors are plugged in correctly (TPS, IAC, Idle switch((name?)).
Also, a really bad vacuum leak will make it rev high and die. It would have to be pretty obvious though.
91' MX-6 LX(T)
: VF12/VJ11 Hybrid/Dual valve springs/Fidanza FW/Maas O2 housing/Mjspiess 2.5 Downpipe/side exit/Walbro 190/Bosh DRI injectors/NGK plugs/wires/MSD B2 coil/ATX TB
: GC coilovers/PGT rear sway/AWR trailing links/Energy endlinks/Dutch FSTB/RSTB