well i'm not sure why the service light would come on every OTHER time instead of EVERY time you start the car but... i do know that on my '96 850 sedan the service light is scheduled to turn on every 750 hours of driving or 12 months or whichever comes first type of thing. it's basically a reminder to make sure you change your oil and do any other necessary service. apparently, the mechanic is supposed to reset the service "clock" when you get your oil changed. but, if you don't get your oil changed at a volvo dealer, the mechanic likely won't know to do it, which is what kept happening to me. if you go by a volvo dealership and explain your situation, they'll likely reset it for you.

hope this helps.