Part of the operation of the ISC is what used to be called a "dashpot", a device on the old carb cars that let the idle speed down slowly, instead of dropping like a rock. If the ISC isn't functional, either because the ISC itself is bad or the ECU has issues, the ECU can't "catch" the idle as it drops. The rpm will overshoot the idle rpm, and often stall. There are several things you should check:
1. Open the ECU and make sure the caps have been replaced. Check the ISC driver chips, IC105 and IC107, to see if either of them look burned. Pull the board and look at the bottom side of the board, underneath the 47uF cap, to see if the skinny ISC trace underneath it is still intact. Also look underneath the cap itself on top of the board. That ISC trace is often the first victim of acid leakage from the 47uF cap.
2. Set the idle speed, as per the VFAQ:
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3. Check your base timing with a timing light, as per the VFAQ:
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I have found that the ISC has a much tougher time catching the idle if the timing advance isn't at 5* or more.
4. Get a cheap logging setup. The software is free. This will allow you to see the operation of the ISC and the idle-closed switch, among many other things that will have you diagnose issues.