[2old4this]

I had the same thing happen with a Jace. It was on an old Network 8000 system and being used primarily for scheduling of VAVs. Quit talking one day. I tried everything I knew and it still wouldn't talk. I thought maybe it had somehow been re-addressed so I brought it back to the office and plugged a port scanner into it that ran for a few days with no hits.
It's not a "mission critical" controller, so I left it on the bench while I went on vacation. You can guess the rest. A week later, I plug it in and everything works. It's been back in service for almost a year now. FM indeed!