1993 Vandura 2500 intermittent no power won't start

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I have a 1993 GMC Vandura, 5L v8 with an intermittent electrical/starting problem. History: It developed the problem a few years ago. It ran fine and was turned off normally, but nothing happened when I inserted the key to restart- no power, no nothing. I checked all fuses, tightened the battery connections, let it sit, checked the accessible wiring (no loose wires found) and checked the battery. When I disconnected and reconnected the positive battery cable it started right up and worked fine for a few days but the problem became more frequent over a couple of weeks. No real pattern- short or long stops, after short or long drives- it just seemed random but became more frequent. Every time it happened, the disconnect/reconnect fix worked immediately but eventually it happened almost every time I tried to start the van. I took it in to the local electrical wizard who said he tried everything he could think of (except swapping out parts) and he couldn't find the source of the problem- but when I got it back, it worked fine for two summers. Now: It started having the same problem a week ago. The disconnect/reconnect fix worked fine the first few times, but the last time it did not respond at first. I checked the connections I could reach and tried disconnecting a few times. Finally there was a small spark when I reconnected the positive terminal and a slight, short, "humming/whirring" noise when the key was inserted- it started right up. Help! Any ideas? Other stuff: I changed the distributor cap/rotor a few months before the first time it had a problem. The vehicle has some play in the shift lever, and something in the ignition switch or keyway is a little worn- the key can be removed while the engine is in running and in any gear- but it has been like that for many years.