I've been having similar problems with my 2001 S40. Problem seems to be when going up a hill (Frequent occurance in my area) or when using the tap-up feature to increase speed. I thought it was a vacuum problem and took it in for service last Friday. Bobby Rahall Volvo in Pittsburgh said they "adjusted" the cruise control and everything was fine, but I it shut off while cruising on the turnpike at about 75 mph the next night. I've seen that other people have been having problems and their dealers replaced a vacuum pump.