Yeah, that part I honestly don't understand whatsoever. If I just started the car, it returns data just fine. It'll sit and idle for a while and still return proper data. But as soon as I start driving, the data gets more and more corrupt until it's not even readable. If I then turn the car off and back on again, the ECU is back to normal and sending proper data... until the next time I drive it (or let it idle for a LONG time, as was the case with the time in the Fry's parking lot).
I don't know enough about the ECU to tell you what's different when you're driving. I really don't.
But the Subaru Select Monitor, in the exact same situation, reads perfectly well...
There goes the "broken transmitter" theory...