1994 Chevy Blazer

Engine Performance problem
1994 Chevy Blazer 6 cyl Two Wheel Drive Automatic

I had a used motor put in my chevy blazer. When I picked it up from the mechanic shop everything was fine untill a few days later when my service engine light came on. The codes read map sensor error, EGR error, and IAC error. I had all of these replaced and picked up my car. The first day everything was fine then it started to shake and pull(like a jumping type feeling) when I came to a stop. As long as I was driving its fine, but when I stop it starts shaking and pulling. It also, acts like it wants to kill but never does. The service engine light came on again so I took it back to the mechanic to find that the error read IAC. I replaced the IAC once again and took it home to find that it started running bad again. Doing the same pulling and shaking. I have taken it back to the mechanic, but I just want to know if anyone has any idea of what my problem can be so that I can have somewhat of an idea of possible problems.

Thanks,
Racheal Powell