OK, hang on, let's get back to basics here...if you're sure there's no vacuum leak, then I think you have a problem with how you've set the TPS/ISC. I didn't see anyone else post on this, so I'll post my method, which is really simple.
The FIRST step is to get the ISC plunger to retract all of the way. Start the car, warm it up all the way to operating temperature, and hold the throttle open with your finger so that it's just off of the ISC plunger, and push the plunger in. You'll see it start to retract. When it stops retracting, pull the connector so it can't move.
Then, slowly drop the throttle back until it rests against the throttle stop screw. Your car should now be idling at 450 rpms--very slowly. Check the RPM parameter for this reading (don't recall which # it is). My guess is that your min. air idle is WAY off--probably around 900 rpms. If this the case, you need to turn the torx screw on the throttle body so that the car idles at 450 rpms. If you can't achieve that idle speed, then you still have a vacuum leak somewhere.
Once you've achieved this idle speed, shut the car off. Turn the key on and enter diagnostics and observe parameter P.0.1. It should read (according to factory specs) around 0. My guess is that this is also off. Loosen the throttle position sensor bolts and move the sensor until this reads 0.7 (this is the way I set mine to avoid throttle sailing--the factory spec is 0). Tighten your bolts, reconnect the ISC, then restart the car.
Now, perform the idle learn, which I believe consists of letting the car idle 60 seconds in Park, then 60 seconds in Drive, then 60 seconds in Drive with the A/C on (not sure, haven't done it in a while).
See if this helps.
If it doesn't--I just thought of this--pull the vacuum hose at your fuel pressure regulator. Or if you have a fuel pressure gauge, check the rail pressure. It could be reading too high because the FPR diaphram might be sticking. If you notice the vacuum hose is full of fuel, then change it, the diaphram is bad. Otherwise, spray a lot of WD40 in the regulator, then put the vacuum hose back on. Restart the car and blip the throttle about 10 times. Sometimes this will free a stuck diaphram if it hasn't actually broken, and will help prevent varnishing on the diaphram.
By the way, nice car...I too am a 92 Seville owner. Do you have an STS? Here's mine...