Just take it to a shop. Pay them a hours labor to find out the problem. If it's simple,D.I.Y.,if not let the pros tackle it.
I spent a whole weekend trying to tackle a similar problem,rechecking connections,swapping parts out.....it was due to a thin rubber band that is used between the connection of the plenum and throttle body that I forgot to take of the old plenum when I replaced it.

I spent probably 12 hours that weekend trying to find the problem,shop found it in 15 minutes.
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-99' Zenith Blue 5-spd...didn't agree with a center divider on the freeway
-01' S Orient Red Metallic 6-spd...money pit...sold to buy a house