Well, I'm beat. Replaced the idler spring. No change. Going to take it to a shop. I can get by taking a chance on a $20 part but will need a pro to tell me if the spindle or clutch is causing the issue and needs to be replaced. Is what it is.
Sharpened and balances the blades myself but knowing me that could be a problem. Middle spindle with double pulley probably should be replaced. Could hear it more so that that other two when I had it apart. They ain't cheap. Will check plugs and coils next time I go up there.
*Update* Everything went together fine. Started mowing and checking belt for wear. About 45 minutes into it I noticed some wear. Idler pulley seemed to be bouncing around quite a bit from the get-go but the whole deck seemed to be shaking after 45 minutes. Could it be the idler arm spring...