We had the very same issue on an 11 with Mer Mail 13 that we have since taken out of service. We changed out every piece of hardware including the chassis, nothing fixed the issue. That leads me to believe it is a software problem, or at the very least software and a specific release of hardware not getting along with each other. We had no acknowledgement of a problem from Nortel, but have heard of others having the same issue too.
Now might be a good time for you to push the issue of going to Call Pilot......