and, I am back
Reconnected the neg cable, fired her up, and I have my gauges back. At least my speedo and tach. Not sure about the fuel gauge or temp gauge as they are in the same position as before, but that could be accurate.
So, I am going to run until it happens again, and then get the battery tested first. I suspect, if bimmers are as sensitive to batteries as I have heard, that if the battery is malfunctioning at all it may be the source of the problem.