Idle on my 99 D1 SD seems to be erratic. I have cleaned MAF, Replaced/cleaned vacuum lines, Replaced Stepper/Idle air, and replaced fuel pump relay. Sometimes idles high like 1500 rpm then 400 rpm or normal 650 rpm. When I put the new stepper it raced to 2000 rpm so shut off and restarted and it went to 1500. repeat and dropped to 1000 then last time normal 650. I assumed it was the stepper just adjusting to its new home.