ATX PSU jumper-trick stopped working. PSU works on Mobo, but not jumpered

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So the power supply had been working with the green and black wired jumpered together (20-pin atx, not 24pin).  Then, I tried starting it up, things flashed on for a sec (leds and the PSU fan) but then shut off.  I thought the jumper was loose, tried again and again, same thing every time.  When power is turned on, the PSU starts for a brief second, then stops.

So I connected it to a mother board and it works fine.  Powers up just like it should, but when I try the jumper, it only works for a split second.  I've tried an alternate black wire on the plug, just in case the one I was using has some issue, but I get the exact same results.  Anybody seen this before and know what's going on?  Thanks

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Right after I posted this, I tried it again, and it worked....    Then I shut it off, waited a moment, and tried again, and it exhibited the same momentary-on behavior.   

Over the last 30 min, i tried a lot of things and discovered that waiting a full 90 seconds after power-off is what's required.  Only waiting for a full minute doesn't work, but waiting 90 seconds seems to do the trick.     I get that you shouldn't be cycling things on and off ever 5 seconds, but having to wait 90seconds to get it to work at all seems to be longer than it should be.  Some kind of capacitor related behavior?    do I need a heavier load than just leds?