Completely Random Starting problem
 

Ok, I have a 2001 Audi A4 1.8 t Automatic with 121,000 miles.
Every so often could be minutes, hours, or days; cool, or hot engine; the car will just crank and crank, but not start. If I walk away for and hour or so, come back it will start like there was no issue. I bought this car 2-1/2 monthes ago for my wife, and have been working on it since, she hasn't even been able to drive it yet because I don't trust it.
What I've Done:
Fuel pump
fuel pump relay
fuel filter
crank position sensor
coils
plugs
collant temp sensor
K&N air filter
seafoam in gas
:mad: I can't figure it out, I have an appointment at the dealer on monday, but would like to see if anyone else has any ideas.
Also, only pending code is po171.(I assume from all of the cranking)
I know there are some smart guys on here who could help, please for all of my sanity HELP ME!

I would start with the battery, then check the starter.
I am going to go with a bad starter.

The car cranks fine, not slow. I would assume that if it were the started or battery, it would crank slower.

MAF problem or vacuum leak? Next time it won't start try disconnecting the MAF wire and see what happens. Be aware that with the auto tranny you may get a TCU code in the process. Make sure you have a VAG-COM or Audi specific scanner that can clear a TCU code b4 trying this.

Ignition switch.

+1

4D4 may have hit it. That is a very good possibility.

where is the ignition switch on my car?

I'll agree, typical ignition switch symptom. They are inexpensive, sometimes as little as $10, but you'll have to remove the steering wheel to get at it. The Chilton repair manual (Passat/A4) has a good pictorial section on this job. By the way, the fault is just burned contacts, so the last time this happened, a couple of years back, I unclipped the switch housing and buffed the starter contacts with #1000 aluminum oxide paper. I know this sounds like I'm being too cheap, but it worked!

The thing you put the key in.:p:D

It's right behind the ingnition lock cylinder. Or maybe think of it as being at the back end of the cylinder.