S10 2.5 efi basic High idle problem

[rcweston]

1988 s10 basic with 2.5 efi and a distributor. 140,000 miles.
For a long time this truck has had a very high idle but eventually slows down. While driving and coming to a stop sign or red light, the truck goes into a very high idle and stays that way for quite some time, then it goes into a slow down mode that takes about 5-10 seconds. At the end of the slow down mode, the truck idles much slower and seems normal.
I know this '88 engine was built when they were doing odd things to stop emissions but for the life of me, I can't seem to see why this idle is so high (above 1200-1300 rpms) but then slows down to about 700rpms after it idles for up to 30 second.
The vacuum hoses seem good, the linkage is not holding it open and the air cleaner is good.
The symptoms are that as it slows down for a stop sign, the engine starts to slow then is increased a little and stays around 1200 for up to 20-30 seconds, then it goes into a 5-10 second slow down period and ends up with an idle of about 700 rpms from that point on.
with a hot engine, the time it idles high is reduced a little, maybe 15-20 second instead of the 20-30 second when it is cooler. It really seems as though the computer is controlling the speed for some reason!!
Is someone familiar with this mechanism and can describe how it works, or maybe better said, how it is supposed to work?
Any insight would be greatly appreciated.
Thanks RC