1998 Oldsmobile Achieva random misfire

I have a 98 Oldsmobile Achieva with less then 70,000 miles and have done everything in the world to get this car to run. When the mechanic hooked it to the computer it said random misfire...but I don't think the "mechanic" knew what this meant, because several hundred dollars later, the car is still not running. What could this be? and how do I fix it?

Thank you