I have had my a4 for about 2 1/2 months now with a few issues and now I drove to work no problems drove home no problems drove to the gas station no problems....
I shut the car off and left the radio on and filled it up and started the car and realized my tachometer and speedometer both suddenly stopped working my tach does a little micro flutter though... Check engine light is on but last I checked it was for an o2 sensor

 

Pull the codes first. Go to an autozone

 

I pulled the codes again... All it said was o2 sensor! I realized if I turn the key on and off until my tach needle lifts up, just a little then start the car it all works fine more info on my car.
1996 a4 quattro
2.8 5 speed manual

 

Any ideas?

 

Seems like its worse when it's hot out

 

Ok when I was leaving work yesterday with the windows rolled up and it being really hot out I couldn't get my tach or speedo to work but this morning with it being a lot cooler it is working fine

 

Ever replaced the ignition switch behind the ignition lock cylinder? They are known to go bad on B5's and they produce some funny symptoms like yours. It's not a big job. $25 part and half hour.

 

Have not had a chance to try the ignition switch yet but I noticed this morning that I hear a repeated clicking sound from my dash almost like a relay that is clicking on and off even after I shut the key off and pull the key out

 

I wanted to post on this topic because it sounds like you're experiencing exactly what happened to me. To start with it sounds like a loud ticking or clicking coming from behind the dash this would be the RELAYS. I went through all of these and found 2 bad ones that stopped the clicking and dash issues. After that my cluster went bad but that was totally unrelated haha but you might want to look into those relays behind the instrument cluster area as those commonly go bad too if you got higher milage. Hope that helps you out a little bit!