1996 corsica

I have a 1996 Chevy Corsica, 2.2L with approximately 130,000 miles. I have had a Service Engine Soon light that has read anything from a Misfire on #3 (which continues to show), to a lean condition. The car runs horribly. There is a slight roughness at idle, but more prominently makes no power when accelerating. I can get it up to speed, but it takes FOREVER. Most recently, when checking for trouble codes, the scanner failed to link up to the computer. We tried 5 times and got the same message ( the misfire on #3 message, however, came up each time). One former mechanic thinks the computer is bad. How do I determine if it is? What other problems could exist and how do I check them? Taking it to a service tech is a last resort, as I really can't afford their fees at this time.