420 SEL - EZL and Incorrect Ignition Timing

[jgl1]

1987 420 SEL (M116.9), approx. 200K miles... measured ignition timing at idle (approx. 1200 rpm) is 30 BTDC, with vacuum; factory sticker indicates 12 BTDC at 750 RPM is correct. Static crank, cam and distributor timing are correct and synchronized; timing chain renewal was performed approx. 40K miles ago. Owner states vehicle was being driven at high speeds for a period of time when engine stumble and rapid fluctuation in RPM was noted. Subsequently, vehicle performance has been off a bit and idle speed is now elevated (1200 rpm).

The engine starts readily and revs freely but is rough at idle and stumbles at low speeds. Spark plugs, ignition wires, distributor cap and rotor are new. Vacuum hoses, lines and connectors are intact.

Since the ignition timing is determined by the EZL, in theory it is not possible to set the timing by turning the distributor. Yet, rotating the distributor counterclockwise results in a substantially rougher idle, lower idle speed and reduction in measured timing; things smooth out... and idle speed increases... when the distributor is returned to approx. 30 BTDC.

Any idea as to where the fault lies? EZL control unit?