My '84 plain-jane waterboxer with Digijet does something very similar...
doesn't die... but idling after a highway run is very poor, and it
stumbles badly pulling away from a stop.  (Kind of feels like its
running lean... stumble, stumble, catch, vroom.)

Troubleshooting to date has consisted of audible verification of idle
switch actuation (good start, Ed!), and just recently, new plugs and
airfilter (old plug gaps were up to .035 or so).  Plus speculation that
something is getting hot in that engine box with no highway air flowing
thru... I even posted a question to the list a year or so ago asking if
anybody's found a 200-300 cfm fan that's a nice fit into the left-side
air intake.

Will be interested in any other comments others may have on this
problem!

thanks,
Ed Mellinger
Monterey CA
'84 Van, appliance white, leaking coolant crossover o-ring...