Audi starting problems

My 2001 Audi A6 Allroad Quattro periodically will not start. I have had replaced the ignition switch, the starter motor, the neutral safety switch and some wiring and it still does it. Turning the key the dash lights up but nothing happens, no clicking sound, nothing. After a time, sometimes a few minutes, sometimes overnight, it will start. But I can’t live with this on any trip beyond a few miles from home.