Based on your description,I suspect that the crank spline is worn and it's moving on the spindle. But stat by eliminating possibilities. The first step ithe easiest, eliminate the pedals by switching to another pair and seeing if anything changes.
Checking for crank arm play on the spindle is tougher. Sometimes you can confirm it by standing on your horizontal pedals, then sitting and gently turning them back a half turn and jumping again. Often this will produce a clear click or movement you can feel, in which case you've confirmed the cause.
Loose crank/spindle fit is impossible to fix, so if so a new crank is in order.
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FB
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Last edited by FBinNY; 11-18-12 at 09:57 PM.