I am not an OSD expert but here are some guesses.
My call sign does not appear at all in the output video (blank area)- but it does in the setup tool.
Maybe the setup tool version does not match the OSD firmware version closely enough.
Maybe the firmware hasn't been compiled with call sign support enabled ? ? ?
The yaw orientation jumps around wildly, but when system is connected by USB to GCS, it is very stable on the GCS display. I am using the external mag exclusively.
I am guessing that when you say USB, you mean the USB on the Revo, not an FTDI (or similar) USB to serial converter (connected somehow to MainPort).
I'm not sure how the code works there, but I can imagine that MainPort data is getting backed up. You could try reducing the amount of data sent (turn unneeded UAVOs off on System page DataObjects <ObjectName> MetaData Modes set to zero). You could try increasing the baud rate if that is possible. I forget whether we now allow 2 UAV data streams, if so, try turning the USB stream off. This may cause problems that it won't connect to USB any more. If so, you can "boot in safe mode on the firmware page" (disconnect Revo power, press Rescue, plug in USB, press SafeBoot) to be able to make changes (be aware that you will see all default settings) ... OR use an FTDI to connect USB to MainPort to make settings changes.
Quad seems to fly properly once enough satellites locked.
If yaw is jumping around, then I imagine you are NOT using any GPS flight modes. I suspect that they would jump around.