Neil, I think you are exactly right. I had this problem on my '88 GL and
finally just gave up. I disconnected that system and it runs and idles
fine without it for general operations. And I simply don't use it any
more. Both the ISV and the ISC (Idle Stabilser Control -box in front of
the right rear brake light) must be functioning for this system to work
properly, keep in mind  however, that ii is the Hall sensor signaling
device on the distributor that supplies the signal that the ECU uses to
fire the injectors.

J0hn

On 5/4/2014 1:09 PM, Neil N wrote:
> FAIK, with that system disconnected, the engine should still
> idle fine when none of those loads are put on it.
>
> FWIW, first thing I'd look for is a vacuum leak(s).