Im guna say it might be a timing or advance curve deal..why would you install a nos dist when MP sells em that are new and better than origionals? I had a similar problem for a while on the duster, changed the plugs , wires, and the dist. finaly found that a prior owner had messed up the dist. i had changed plugs n wires just cuz they were old.
If you want i could stop down some eve and we could look at it. Ive ben meaning to swing by randys to get my 4.56s, an drop off his 3.91s an 4.30s. i would like ta see your car and meet you and your dad anyways some time.