So after 3 months of high idle insanity and 3 stepper motors the issue is resolved. I switched out a new stepper motor for a second hand one on ebay and the idle dropped to about 200rpm. I didn't know it was possible for the truck to idle that low but it did.

This may not sound like progress because the used stepper motor was not functioning correctly BUT it immediately negated all the possibilities of vacuum leaks etc. etc. The very helpful ebay seller offered to replace the stepper motor and out of the goodness of his heart included a used TPS. So i just thought what the heck and replaced both and it is sooooo wonderful to have the truck idle around 700rpm.

Thanks to all for your helpful suggestions.
Alan
98 S1 Disco V8 SFI