We're running the w2k advanced server for our post offices and MTA and have had no problems like you speak of, even during the beta period. Ours has been up and going since last November and haven't seen any problems at all on the post office/MTA side.
I would recheck that your PO and MTA SP4 patches are up to date. Also make sure you are running the latest client version on your BES server.
You could also try turning off various PIM features such as address book, memos, tasks, appointments, etc... and running it barebones for just email and see if that fixes the problem. If it does, re-enable the services one at a time to see if any of them will crash it. It will at least give you a starting point to work from, or allow email until the problem is found and resolved.
I don't know that this would help, but you could try re-installing the BES server. The database won't be touched, so the users you already have activated will just notice an outage, but won't have to re-activate when the server is brought back up.
It could also be possible that a single user is causing the problem. If you set the mdglobalconnector (I think that's what it was called) threads on BES to one, you can monitor it and see if it is crashing the post office on the same user everytime or if it's random. If it is a single user, you could disable that user and see if it still crashes. Re-activating the user, or running gwcheck on their groupwise account might fix it.