Don't know if this will help, as I'm sorta new to Fieros. On my TPI engine, I use this procedure for setting the IAC and TPS. They are sort of an incremental adjustment- adjust one, and the other can drop off a bit- so you need to repeat is several times to get an exact set. Other's more knowledgable can tell us whether this will work on the Fiero, but they should be pretty close.http://www.thirdgen.org/tpimod2
But it does sound like a large Vac Leak causing most of your issue- keep looking
'87 GT in process!