Pete, that's very odd. I have a hunch, though... try disconnecting your OPII from your network for a day, and see if your thermostat behaves during that day. I'm thinking that if something on your network is jabbering, data link layer processing in the OPII may be tying up its CPU to the point where task scheduling is being thrown off. I'm not sure about this, but I'm guessing that the OPII does all of the RS232 framing and start/stop bit processing for the thermostat bus in software, and if there are tasks which are overrunning internally, it could screw up the thermostat bus, and that would also explain the time of day anomalies you're seeing. Of course that doesn't explain why it works with the RC80... but maybe the RC80 is better at ignoring transmissions with bad framing.
(Also... you have secured your wireless network, haven't you?)
(Also... you have secured your wireless network, haven't you?)