Car won't start intermitttantly

My daughter has a 1999 Saab 9.3 turbo, approx. 100,000 miles. It will start and run fine for days then after being switched off will not start. After towing it to a Ssaab repairman twice it will then start and run fine and show no fault codes. We drive it home, it runs another day or two then the same scenario. It turns over but will not even fire occasionally. The mechanic is stumped. Is it possible there is sediment in the tank shifting to the fuel pickup? Help !

Guesses - bad starter relay or connection? Bad fuel pump? Intermittant connection somewhere in the ignition circuit? Sediment is way down on my list.