… I’d really appreciate it. BTW, the car still runs great with plenty of power. It’s just this weird dip in idle that I’m trying to solve, but at least the problem isn’t intermittent anymore. ….. Yes, the computer does regulate idle – but base idle is set mechanically at the throttle body. The lack of speedo signal to the ECU is indeed the culprit in this case. Odd though that his speedo has a dead odometer and it causes a no signal to the ECU for VSS. … …

See the article here:
240 Strange Intermittent Idle Issue 93 240 Auto – Turbobricks Forums