My 3.9V8 Disco-1 idles erratic sometimes (cold & operating temp). Most of the time she's fine. The temperary high idle after startup is fine, and when the air-con is swiched on (operating temp) there's no decrease in idle speed, which means the stepper is working. When blanking off the air-bypass hoses to adjust idle speed with the allen screw on the throttle-body, she won't idle at all, doesn't matter where I set the little screw.
Pieter