I took my son,s 81 GMC pickup with a 250 in line six in to get certified and safety checked.It has 75,ooo miles on it.I asked the mechanic to tune it up as it had a very slight miss to it,but lots of power.He changed the distributer,rotor and plugs.Now it idles rougher than before.The mechanic claims those motors were always rough running.What should I do now as I don,t want to cause any damage driving it like it is?I heard those engines were very long lasting and dependable.