I have an 89 RS TBI 305. I just rebuilt it and it only has about 80 miles one it. Today I was driving and the service engine light came on. I was on the freeway and nothing seemed to have happened. But I got off the freeway and turned into some parking lot. When I slowed down and stopped the enigine was kind of pulsating, from about 700 to 400 RPMs. I drove the rest of the way home and in the drive way it was doing the same thing, but it got worse and now it dies when it is in gear. When it's in park or neutral it has a nice even idle. What could be causing the problem???

Also, I was reading the repair manual and it talked about the trouble codes being stored in the ECM. What does this mean? How do you find out what the trouble code is? Please help!!!