Neal,
I've never heard of a Hall-Effect sender working OK at idle and failing at
higher RPMs. Usually the heat gets to them and they quit until they cool
off. But, anything is possible. Get a can of freeze from Radio Shack to test
this theory. I do like the suggestion of inspecting wires underneath the
distributor. Be very careful removing the plug, the insulation will be
extremely brittle.

How old are the spark plugs and associated wires, rotor and cap? I had a
plug wire fail inside the metal boot that covers the plug. Current was
directed to ground instead of producing a spark.

Good Luck,
Ken Lewis
http://neksiwel.20m.com/