1986 Dodge, 318 motor, 4 x 4, auto, A.C.

Turns over good.  Will not start.  No spark to plugs.

Take coil wire out of distributor and ground it.  Turn pickup over a lot or just a little with key.

Plug coil wire back into the top of the distributor and she will fire right up. 

It can do it twice in a row or it can do it once every 6 months.  Wet, dry, day, night, hot , cold,  make no difference.

What is causing this problem?