The leads were prone to literally "break down" on cars that had leads. The insulators that connect to the plugs drop to pieces. Make sure that you get all the debris out of the plug recesses before removing the plugs/leads or it ends up "down the plug hole" and into the cylinders when you remove the plug. A length of copper pipe sellotaped/duct taped on the end of a vacuum hose is great for cleaning it out before plug removal. ( a PITA to check on the rear bank of an early V6. A torch and a shaving mirror come in handy ).
A new set of plugs and leads from DMGRS cured my miss.