start runs fine then wouldn't start ?
 

My 86 911 Carrera 3.2 just had a top end overhaul. for a short time it started and ran quite well. Recently it all changed. Now when it starts all seems fine, I take it for a ride shut it off to gas up or grab a bite, and I have to hope that it will restart to take me home. When it don't start it gives no indication that its trying to start. The engine turns over and even builds druck pressure. I just had the ECM totally gone over but, all was well with it. installed a new coil, it was not the answer. Also the DME fuel pump relay was also replaced. Its an intermittent problem, making it difficult for my very capable Porsche mechanic. Any suggestions ?

Has your mechanic checked the ignition switch ? it could be that its losing contact with the run position contacts after the switch get's a bit warm.
A...

There is a thread from 2011 where people were replacing both speed sensors with good results after replacing DME relays, ECU's and all sorts of things.

Link:
http://forums.pelicanparts.com/porsche-911-technical-forum/614582-911-carrera-3-2-no-spark.html

Speed and reference sensors.... replace 'em. Do the O2 sensor while you're at it. Look at the wires... I'll bet the insulation is breaking off by now.

When you say no indication that it's trying to start, does it crank?

Agreed on the speed sensors. That's a real fun job too. Sensors corrode and get stuck in their respective holes.