When my 1986 420 did this, it was the crankshaft position sensor. It's on the upper driver side of the bell housing, as I recall; a green wire goes form the ignition module on the driver's-side inner fender. It wasn't that bad to replace, but I recall having to do it mostly by feel. Anyway, replacing it cured my problem.
While you're at it: there is white silicone paste underneath the ignition module, which helps carry heat away from it to the steel inner fender which acts as a heat sink. You will probably find this to be dried out when you remove the module. Get some at Radio Shack and clean the old out and replace it. It will help the life of the module.