BB Bold 9000 :: Mute Button Slowly Stops Working (multiple Devices)

I have a Bold 9000 on the AT&T network running v5.0.0.464. I'm having a problem with the Mute button (on top). I use the button to put the device into sleep/hibernation/whatever. I have had this problem with three devices (all 9000's) and have had each replaced because I was told that it was a hardware error by the AT&T rep. However, I have done some testing on additional devices (corp. environment) and have been able to reproduce the issue, or more accurately stated, I have been able to observe the issue occurring on each device in my test group over time.

Basically, when the device is new, the button works very snappy. Over time, looks like 2-3 months, the button begins to show intermittent issues and takes longer and longer being held down to activate the function (starting or sleeping). Eventually, this function has quit working on every device I've tested (out of the box devices with no apps beyond the delivered ones installed). Removing the battery repeatedly and on a regular schedule of once a week has not had an impact on the issue during testing or in Prod. Being new to supporting these devices I looked around and did some searching but haven't found a solution that has worked for our situation.