I have a '99 A8. Just recently I've had this problem where the brake light and abs light come on whenever it they feel like it. I've changed the pads and I'm still having this problem usually in the morning. Sometimes it feels like the caliper is hesitating and then biting down on the rotor. When they heat up they squeek real bad and loud. I need help from someone who's experienced this. Do I need to change calipers or is there a bigger problem?

 

I think there is two problems, one electrical and one mechanical.
You need to have the codes pulled from the brake electronics control module, if you can't communicate with that module, then you need a new module.
The mechanical problem is probably your brake pads are worn out.

 

https://www.audiforums.com/forum/general-tech-50/abs-esp-lights-dash-12484/