cars not running right maybe you can help

[avguy]

... i never really noticed it til i put my aftermarket muffler on and i can hear it and feel it slow down and speed up. ...

Whenever I get a brand new problem (any, not just cars), right after I did something else, then I'm going to believe that's the cause of the new problem until proven otherwise. Now in this case, maybe it's not the new muffler itself causing the problem. Perhaps something like a vacuum/air line that didn't get reconnected tight, or something getting inadvertently hit and knocked loose. Might be worth checking everything in the area where you did the work.