80 Quattro Starting Problem

[sembai at verizon.net]

My husband's 1990 80 Quattro has developed a strange starting problem.  One evening a couple of weeks ago he went to start the car as usual.  It turned over and ran roughly for maybe two seconds, then died.  He tried a second time, it did the same thing, but for a slightly shorter period.  On the third try, the car didn't turn over at all.  He waited an hour and tried again, and it started right up.  The next day he left the car home.  I went out and started it a couple of different times without any problem.  I also put a battery charger on to make sure a low battery wouldn't be a problem.

Concerned about possible looming trouble, my husband sent the car over to his Audi mechanic.  The shop checked out the cold start valve, replaced a fuel injector sensor, and checked out the battery.  They may have done a few other things as well, but that's as much as I know was done.  While there, the problem never reappeared.  A relative picked up the car without any trouble.  When my husband picked up the car from the relative later that night, the starting problem of course appeared.  The relative returned the car to the shop, which checked out the car and couldn't replicate the problem.  My husband picked up the car after hours last Friday and drove it home, since the shop couldn't find the cause.  Saturday afternoon he tried to start the car, and the starting issue reappeared.  We left the car until Monday afternoon, when I tried to start it and encountered the same problem.  Today I went out late in the afternoon and couldn't get the car to turn over at all.

At no time in the past couple of weeks did any dash warning indicators come up.  The only other thing I can add is that the car is 15-years-old w/ 170,000 miles on it, so it doesn't make sense to perform an expensive fix.  The shop was very good about not charging for the diagnostic work, just for the sensor replacement (which it apparently did need).  

Any thoughts would be appreciated.  I'm not a mechanic or an engineer, so please keep that in mind if responding.  Thanks!

Holly