audi 80 loss of power

after about 5 to 7 miles my audi loses all power.if i stop the car for about 20 seconds or so and then restart it the car runs fine for the rest of the day no matter what distance i do.i have changed all ht leads,spark plugs,rotor arm,dist.cap and the distributor itself and the coil i have also changed the inline fuel filter.it always seems to happen after the same distance each time.it is an older car [f reg] with 179,000 miles, i have also changed the ignition switch unit and problem is still present. i have started to think about the fuel system but i would have thought that the loss of power would be more random. because the power loss happens every morning when i go to work but does'nt happen on the way home i wonder if the cooler mornings has any thing to do with it. ANY SUGGESTIONS as i am baffled.
cheers
gabby