John
1993 is on the cusp of the changeover from non-cat to cat systems. Do you have cats? If positive then I would suspect one (or both?) of the lamda sensors. Another possibility (as per David above) is an air leak in the exhausts which is causing the lamda sensors to request the eco to overfuel.
First the "silly" question, you are sure that you don't have a sticking throttle cable?
Another question, is the ECU "chipped" at all with new fuelling maps? The Tornado chip is the most common, and many +8s will be equipped with this modification as it helps
My 1999 car (with cats) has always had a slightly high idle for a couple of seconds after lift off , but it does settle down pretty quickly thereafter , although bringing the car to a halt will accelerate the rev decline.
