T46G Slowing Down Until Lockup

Hi,

I've got a handful of T46G phones all running firmware 28.80.0.95. Two of them have the expansion module on the side. One of the phones (with expansion) seems to have a recurring problem where the phone get more and more sluggish until it is unusable. This is the only phone having issues and a factory reset didn't help.

Are there any known issues like this? How can I narrow down the cause?

Thanks,
Kevin