Are the iPockets getting unplugged? I had a similar issue at a hotel where the PMS server was under a desk, and the iPockets would get unplugged all the time when the front desk people stretched out there legs or moved stuff around under the desk. A little 88T tape in the right place fixed that problem.
Also check Form 4, Option 45 (Disable PMS Logs) and make sure it is Enabled, and Form 35 and make sure PMS is NOT set to Guaranteed.
This will only effect the logs though, if the PMS is going down, there is a reason for it. If both are going down at the same time (VMail & PBX) then something in common is causing the issue... the SX200 is not likely to be the issue.
The best way to determine this is to connect to the iPocket with a laptop and use Mitel's PMS Emulator software when the link is "down", just unplug the cable from the PMS system and connect it to the laptop and run the PMS Emulator, if it works fine, it's a PMS issue, if it is still failing then check the iPocket setup.