Key word is intermitant. Fixed 1000's of electrical equipement and nothing infuriats me more than the problem that won't show itself till your doing somthing else. Then try to imulate it!

+1 Intermitent problem(s) can't be found without process of elimination which unfortunately requires throwing parts at the problem.Some of these parts are what I consider maintenance items as they do fail due to age.

I'm with anyone who suggested:
1) DME (Fuel Pump) Relay and related contacts
2) Ignition Switch

ditto.... check you reference sensors mine does the same thing once in a blue moon,either the connections are corroded or they are failing.DME would be you other choice,check connections.let me know what happens.

UPDATE: It finally failed for the mechanics! They are thinking probably a reference sensor or a DME. Something that regulates power to the coil. They didn't get the chance to test it when it failed because it started up by the time they got the equipment out to it. At least they know I'm not crazy. I'm pretty sure it's not the ignition switch because I can beat on it and wiggle it and it doesn't start. It always comes down to waiting. We'll see if they can't get it to fail one more time, long enough to test it. I trust these guys. There were 2 early 911's with the engines pulled out when I dropped mine off, so I no Porsche owners trust them with their cars and they have experience there. I'll keep you guys posted.

The ignition switch provides power to the coil. The DME grounds the coil to fire it.

Your ignition switch "test" is bogus.

It would be interesting to see if the car is loosing the injector pulse as well as spark.
Since you made no mention to black smoke when the car does re-fire I would suspect it is.

Another vote for the DME (Fuel Pump) Relay. Relays usually go bad slowly because of rusty / weak contacts. It's easy enough to just put in a new relay. (If it isn't the relay, you can keep one as a spare. They do go bad.)

When it fails.... tap the dme.

More than likely bad soldering point on the ignition driver....

Motor Meister has probably 10 times as many cars with their engines pulled, people trust them -- doesn't make them trustworthy... FYI.

Glad they were finally able to replicate the problem. Now they can get to repairing it. Way I tested for bad speed/ref sensors was with an oscilliscope... Let the car run and wait for it to show irregular spikes on the graph.

Not a lot of people know how to correctly operate an ossiliscope. If all else fails use, there is always driving arouns with a volt meter hooked up. Black lead to ground, place the meter on your console and hook the red lead to whatever point you want to monitor. DC volt scale and watch. Positive side of coil or negative. Positive will maintain 7-12 volts constant. Ground will be 0-.4 volts and float to 7-12 if the conection is off. Same for any other point monitord.

I know a local Porsche shop with a very respected owner/mechanic. I'm in there one day waiting for Oly and watching a mechanic wack on the side of a control arm to pop out the ball joint (BMW) and low and behold, he hits the fender square. Wipes off the smudge and keeps working. You think he told his boss or the owner?

LOL paul, remind me never to go there!

Which shop is this... come..on do tell... .

Hopefully you figure out what it is, as there are the several possibilities that have been stated here. My car behaved like this for about 2 weeks while my reference sensor was on its way out, but luckily for me it decided to completely die one day... What color/yr/model 944 do you have? I will have to keep any eye out for you on the roadSmileWavy

It's a 1987 Silver 944 N/A. Do you live in the city or the suburbs? I go to Hamline, so I'm in Midway, St. Paul.

If the ignition switch was bad, would the car die while idling and fail to start, or would the problem only occur while trying to start the car?

Find and read then perform the five minute test.

SoCal- Found it, read it, and now I'm gonna take my car back from the mechanic this weekend and see if I can't track this down. Thanks.

I'm assuming this is the test to which you refer:

Well the shop was I believe Furtmier motors? here in Bridgport. Part of Kitchener. Not the freindlyist guy either. Stopped by one day after buying a lot of parts, to ask a general question with the spun #2 bearing in my hand. Looked at me like, what do you want, pay me for the work. Last time I go in there.

Part of it. Once these findings are made there are further check points. Most of the time it's the DME relay and/or the sensors.

Got my car back from the mechanic today. They found a burned out pin on the DME; one that would interfere with spark to the coil. I have a new pin soldered in and it didn't cost an arm and a leg. I'll hope this solves the problem, but IF it fails on me again, I'll run the test and we'll go from there. Here's to hoping...