I may be able to help here.

I had the racing idle problem in 1997 in my '86 Westy.  I finally traced it
down to excessive heat from a transistor in the idle control unit (ICU).  I
found the unit would work OK if the ICU circuit boards were removed from the
plastic case, allowing greater cooling.  The fix in the archives was to
replace the discolored transistor with a higher power transistor with a heat
sink that would handle the current without generating too much heat.  This
worked for a while until the idle began to fail again.  I found drilling a
hole in the ICU case and sticking an even larger heat sink for the
transistor out of the hole for greater cooling worked until I removed the
engine for a conversion.  The components in the ICU were not burning out
initially but something was becoming erratic because of the heat.  I think
the primary problem was the idle control solenoid began drawing too much
current causing the current switching transistor to overheat.  The heat
inside the ICU caused the secondary problem of the shutting down the control
circuit, resulting in the engine racing at idle.

I can provide pictures if anyone would like to see.

The first time this happened was in a parking deck.  The Westy had an
automatic transmission at the time and I had a broken arm in a cast.  It was
real scary.

Ed