Diamond mutes calls after random amount of time

[sastraxi]

I swapped in my Diamond a couple weeks back because the USB port had warped. Ever since then I've had a problem: something or other randomly mutes the sound! It happens when I'm in calls... anywhere between 2-10 minutes in, the phone will go silent. The other side can still hear me (tested it and it's lasted at least 2 minutes in this capacity) but I cannot hear the other side. Soft-resetting clears it. AFAIK this has happens on multiple different ROMs, the stock Telus one, OMJ and Mystery.

I've tried upgrading the radio, but that doesn't help. Any clues!?