I've just run a 2500 mile trip in my '99 A6, a couple of niggling things with my A6, while driving the brake light comes on then the ABS light comes on and the warning beep sounds, comes on for approx 3-5 seconds then goes off, looked in manual and couldn't find anything. This happens every couple of hours.
Another small issue is that the temp guage will register to normal temp, then will return to C stay there for a while then go back to normal temp, my thinking is that this is a bad temp sensor!
Please post with comments / help / advice

 

ABS is sensing something wrong... pull the sensors and clean them AND the 'toothed ring' in the hubs. Brake dust builds and fills the gaps in between the teeth and makes the sensor think the wheel is not turning. Check the brake fluid for color and level too.


Temp thing could be the thermostat.. how long since it and the TB belt were changed?

 

The ABS light is most likely caused by the brake fluid level. If the level drops, both the brake light and ABS lights come on. The on and off nature suggest that it is just about at the minimum. This is caused by brake pad wear, you are probably not using fluid. You should check the pads, as you may be ready for new ones.

I would agree that the sensor is most likely the culprit in you temp gauge. Also consider a thermostat if the sensor does not cure it.

Siricospeed

 

You need to pull codes from your A6. My ABS/Brake lights would come on, followed by 3 beeps. Turns out it was a faulty ABS controller.

I have a VagCom, and I pulled these codes:
18014/P1606/005638 - Rough Road/Engine Torque Signal from ABS: Electrical Malfunction
01203 - Electrical Connection between ABS and Instrument Cluster