*If* you own prior observation is correct (spark to cap, no spark to plugs) it is exactly as Jeffsr stated. If not however, one other possibility is the electronic ignition box itself. I am not sure if they were still in use on your model in 1984, but it seems very likely that they were.
They are usually very reliable, but I did come across one that was intermittent, and the symptoms are pretty much as you described. Also check the green wire from the distributor leading to the ignition box for continuity & good contact on each end. This wire should be kept away from the high-tension leads as well, just in case someone re-routed it. (Even though the wire is shielded, the high voltage wires theoretically can cause an induction current that would foul up the signal. This will *not* be your primary problem though.)
Yet another possibility lies with the ballast resistors. It is possible, yet very rare, that the resistance coil has fractured. This will allow voltage to pass when they are *just* touching, yet any vibration, or the heat that is generated, can cause it to shift, thereby opening the circuit. The other (not so) long shot is the coil in the hall-effect unit itself, located inside the distributor.
Please let us know what the problem was when you find out.
[Edited by RunningTooHot on 07-20-2001 at 06:27 PM]