Well, you've either had the incredible bad luck of having two Zunes with the same problem, or something you're connecting your Zune to is damaging it...maybe dirty power coming from an outlet is damaging the Zune? You didn't explain how you're charging it, but perhaps try charging it through a surge-protected power bar...
I'm completely guessing though, this is a bizarre problem that I haven't heard of before.
__________________ Want to contact me personally? Use this. Want to read my personal blog? Check it out. Want to follow me on Twitter? Here you go.