1998 Audi A4 Starting issue

Hi Prowler53. Welcome to the forum. You have the best possible situation. You can make an intermittent problem act up at will. When it doesn't crank, do the head lights work? Can you hear the starter relay click under the hood?

Something is flexing and possibly tugging on the battery cables. Also look at the small black wire from the negative battery cable where it bolts to the body. Be sure all of those connections are clean and tight.

Caradiodoc

If the head lights still work, that rules out that small black wire I mentioned. Next, you'll need a test light or a cheap digital voltmeter. Well, ... Wait a minute. I was going to have you measure at the starter, but if the large cable was loose, you would hear a kind of loud clunk from the starter. Instead, check the smaller wire on the starter solenoid. There should be 12 volts there when a helper turns the ignition switch to "crank". Check that wire to be sure it's clean and tight. If there's no voltage there, look at the starter relay and the wire from it to the starter solenoid.

You might have the best luck finding this by wiggling and moving wiring harnesses and other things while your helper holds the ignition switch in "crank".

Caradiodoc