85 turbo - idle going crazy

Idle speed keeps fluctuating for no reason, way overspeeding sometimes. Tried
swapping in a new throttle position sensor as this worked on another one of
these cars I had once, no change. Also when car is doing this when first cold,
the speedometer starts reading off random numbers (??) but this seems to go
away shortly. Am not getting any power loss light nor any stored codes. It
definitely seems to be the computer telling the idle motor to increase
erratically - if I unplug the idle motor, idle stays steady forever until I
plug it back in, in fact I'm driving it like this now. It seems to vary
between normal and various overspeeding, it is not making it go too low (not
stalling). Any suggestion on what to check or change next? Could it be either
of the sensors in the distributor? I changed the rotor but thats it.
thanks!...Mark

 

Before I spend $ on the speed sensor...if I unplug the reference
pickup, the idle drops to a steady level. Plug it back in and idle
jumps back up and starts hunting up and down again. Does this
indicate the problem might actually be that pickup? thanks! Mark

 

Correction, I am unplugging the sync pickup, not the reference.

 

AFter changing the distributor pickup I think the problem turned out
to be some crud in the throttle body plate and/or orifices. I had
scrubbed it while back but apparently some more must have built up.
Idle must have been simply adjusting to fact the crud was built up in
that area. Cleaned it and idle is steady as a rock now....Frenchy