s40, I'm pulling this right out of thin air but....

Have the techs. done anything with the electronics?
With all the electronic goodies these cars have under the hood I'm thinking maybe an ECU issue or battery, especially with your comments on "dash lit up like Christmas tree".
I know this sounds like something any tech. would do but it might be worthwhile to reseat everything in the computer box, every connector. Try tossing a new battery in it; see what happens!