Most parts stores can only check ODBII systems ('96 and up).

Did you remove and clean the IAC.  They sometimes need to be
removed and soak the valve poriton in solvent overnite to
break some of the gum loose.  You may also want to re-clean
the throttle body using an old toothbrush to work all the
gum lose aroung the throttle valve.  Another thing you
should check is fuel pressure.  That vintage had an
ovvasional pressure regulator failure for which they were
recalled.  If you pull the vacuum line from it and find
fuel, it is definitely bad.  Another indication is burning
eyes when you are close to the exhaust.  This usually
results in poor fuel mileage.  Another problem with that one
is the tendency for the gasket between the upper and lower
intake to leak.  It is easily replaced but, you must torque
it properly as they are very sensitive to this.  A leak
along this gasket can sometimes be found by spraying carb
cleaner or a similar material along the gasket while looking
and watching for a chenge in idle.  I do not suggest ether
as you can light yourself up easily and then it won't much
matter how the engine idles.  It can also damage the engine
if you have a large leak and therefore get a larger slug
sucked in.  Unfortunately, the leak is usually on the right
side where it is hard to ID.  If you decide to replace that
gasket, there are a couple of gotchas.  Someone here can
fill you in if you tackle it.

Lugnut

Idle Air Control (IAC) will likely be the problem. I think I paid about
$50 for mine. They may be cleaned if you want to try that first.

Gary