Audi 100 S shuts down mysteriously on highway

Hello all,
I was cruising at 70mph, when my tach (rpm) started to oscillate with
a clicking sound, and the engine shut down (rpm went to zero and I
lost power steering). all warning lights came on. I pulled over the
emergency lane, and came to a complete stop. The clicking sound still
persisted until I turned the ignition off. I restarted the car
immediately, and it started without any problem. After 20 miles or so,
it happend again.

I took it to a shop, and they pulled the codes, and there were a bunch
of them.

00545 - Engine transmission electrical connection open/short circuit
to ground - intermittent

00518 - throttle position sensor G69 short circuit to ground -
intermittent

00638 - Engine/transmission electrical connection 2 open/short circuit
to ground - intermittent

00532 - Supply voltage (B+) too low - Permanent

They cleared the fault codes, and I set on a 1500 mile long drive. The
first 800 miles were perfect, and the car died mysteriously again
(with the same symptoms as above). After a couple of seconds, i
restarted the car without any problems. This seems to happen only when
I exceed 60mph. The car died again a couple of times in the next 700
miles when I seemed to go over 60mph. At the end of my trip, I had the
mechanic pull codes, and there were no codes !!! They were not able to
replicate the problem as well :(

Any help will be appreciated.

Thanks.