The cruise control switch, vent valve at the brake pedal may have bad contacts. This may cause the cruise control to shut off. It is located next to the brake light switch at the brake pedal. I followed the diagnostic instructions on the Bentley CD. This switch should have 0 ohm with the brake pedal released and infinite ohms with the brake pedal depressed.

 

I've also had my turn signal turn off the cruise control. Even weirder, it sometimes will shut down if I adjust my power seats.

 

My cruise control doesn't work at all. 98 A6 2.8L Q.

What is the fix for this?

 

I don't have a solution, but I noticed that my "resume" is not working anymore. I can set the cruise using the button on the end of the stalk and it holds speed no problem, but if I brake and then try to resume using the slide switch nothing happens. Sliding the switch to "off" does cancel the cruise, so it's not like the slide switch is completely dead. Maybe in the six months my A6 was stored in the garage I forgot how this works.

 

The cruise control switches on the turn signal lever may be cleaned per http://forums.audiworld.com/a4/msgs/1996882.phtml .
After cleaning, I used silicon dielectric grease on the copper circuit board and on the roller. The circuit board had a layer of dried out grease and tarnish, which caused high resistance. You will need a T-6 screw driver from Radio Shack.

 

So, if all of the tests for the switches and the vacuum pump pass, as outlined in the BentleyPublishers.com Audi shop manual CD, then the cruise control module may be bad. So I found a local yard, which had a used module using: http://car-part.com/dealersaz.shtml
You can also get a rebuilt module from: http://stores.ebay.com/ABS-ECU-ECM-E...QQfsubZ8QQtZkm
You could also buy a new module.