My car pops up the service active handling code. For me it happens mostly when the outside temperature starts dropping. You can actually check the codes to see what is causing the fault. In my case, it is the front wheel speed sensor (ABS sensor) which requires the entire spindle to be replaced and the steering wheel position sensor which is awesome because you can not buy a new sensor, anywhere, for the 1999 and 2000 C5.
I have not cleaned my grounds yet. Once I do that, if it still pops up, I'll replace the spindle, if it still pops up after that, I'll either unplug the steering wheel sensor and just hit reset everytime I drive it (Yay!) or look for a used steering wheel column to install as you can not remove the sensor without damaging it.
Let us know what you find!