ABS control module failure is fairly common. Search on
"abs module", below, to the right. You will find a lot of
posts including options for repair or replacement. If the
module is failing your brakes will continue to work -- you
just might not have ABS.

Ignition switch problems can also cause this. A good general
discussion on other possible causes is at:

http://www.volvospeed.com/Repair/abstrax.htm