I would recommend you rent an oscilloscope and check the output from
the reference and speed sensor first.

Try here also  :http://frwilk.com/944dme/

Real important !!

My 1987 was driving me nuts with intermittent problems just like yours.  I
swore it was the coil also, and it's almost never the coil.  My car would
just suddenly die on the highway, and then kick back in and run.  Sometimes
it would be like impossible to start.  A good check if the condition is like
a no start, is to smack the brain - and voila my car started.

It's the DME BRAIN - the car's computer - I BET ON IT.

I borrowed a brain from a friend, and my car ran great.
So I opened my car's computer, (under the passenger footwell carpet) - I
resoldered the larger soldered connections in there and the car never ran so
good.

It seems like many of these brains and the cruise control brain too, have
cold solder joints.  Shame on you VDO!  (VDO makes gauges and stuff for
ROLLS ROYCE no less - and this is DEFINITELY a manufacturer's defect.  Also,
since it affects the pollution controls and is contributes to a VERY
DANGEROUS condition - stalling at highway speeds - all of them should be
recalled and paid by VDO - not Porsche.

So free flow the solder yourself and fix those control boxes.