I put a post here last week about a problem i was having with my 91z28. the engine was missing and the tach was jumping all over the place at idle. Well, I looked eveywhere for a problem and 3 days later I found it. I have a brand new MSD dist and figured there was no way it was the cause. well, it was. I took the cap off and noticed that the pick up coil was really rusty looking. well I figured i should clean it to see if it would help and when i took it off i noticed one of the 2 wires that connects to the ign module was cut in 2. apparently, it was rubbing on the dist shaft and evenually wore down and was arcing on the dist. this was causing the engine to miss and the tach to jump. I fixed the wire and it brand new again. i was pulling my hair out but im glad it fixed now. thanks for the advice from some of you.