83 5E engine dies on highway

[Peter Wijtvliet <Peter.Wijtvliet@budget-emea.com>]

While driving my 83 Coupe 5E this week, I have been faced with my engine
dying on me twice and almost a third time.
What happened the first time was that I was driving along the highway at
about 90 mph when suddenly my engine just 
cut out. I left it there -after which it got towed away and cost me a
fortune to get back- thinking my fuel gauge I had just
installed was giving a wrong reading and I had thus ran out of fuel. (Quite
hard to judge with a faulty fuel gauge AND broken odometer).
While filling up the car I couldn't get more than 30 liters in the tank so
it couldn't have been empty. Ignoring it, it happened to
me again two days ago when after driving an average of 80mph on the highway
it died on me waiting for a traffic light.
When this happens she is totally impossible to start, the starter engine
does work, however just refuses to start. The last time this 
happened I decided to just sit and wait and think what the prob might be. I
tried starting her about 7 times over a period
of time of 25 minutes and refused to start. All electronics work; just not
the engine. then after 30 mins. I tried and she started
asif nothing ever happened. 

Anyway; I tested her yesterday after giving her a full service. While
driving 100mph, what she did this time is without doing anyhting
suddenly the rpm shoots up from 4500 to 7000+rpm and falls back to normal,
then falls down, cuts out and restarts (by itself)
asif nothing happened.

This is not as smooth as it may seem from the above... eventhough it goes to
fast to realize what happened.

Anybody any tips/ideas?

Peter

UK/NL