Idler tensioner spring keeps breaking

[HD75_mower]

I have a John Deere HD75 walk-behind mower with a 48" deck. I bought it about 5 years ago and it's worked fine until this summer. The spring that tensions the idler for the deck drive belt (under the power unit, not on the deck) broke near where the small end hook is formed. I figured the spring was old so I got another one. That one lasted about 30 minutes. I figured it was defective so I got another. Same thing. So I started looking for reasons for high vibration of the idler pulley arm. I found about 40 mils of runout on the jack sheave on the deck. So I replaced both the jack sheave (one pulley driven by the motor, other pulley drives the deck belt) and the spindle it mounts to. The vibration definitely decreased but I broke another spring after about 1 1/2 hours.I figured I must have a piece of chip stuck in a pulley somewhere but I looked at all the pulleys and found nothing. I took pretty much everything apart at this point, inspected the three spindles on the deck (all fine) and re-greased them, checked the tensioner pulley on the deck - seems fine, took the deck tensioner arm off, cleaned and repacked its inner sleeve, same with the idler pulley arm under the power unit, and took the PTO clutch off and inspected for obvious pulley distortion or bearing issues. I even greased the stud that the idler spring hooks over. I'm out of ideas. The only thing I can think of is intermittent catch and release of the PTO clutch but it's not making any noise that would indicate this and I can't think of a way to test for this. The vibration is still about the same and I assume that I'll get an hour or so out of the next spring. They're about $18.00 a piece so maybe that's not a bad rate for mowing but it's really inconvenient! Any ideas?