thats normal operation, although only one minute of lean burn sounds like you are on the ragged edge of the lean burn window. try 60 mph on the same route and see if it stays in l/b longer. btw, the drop out of l/b is to purge the NOx cat converter.
see:
http://www.insightcentral.net/encyclope ... lytic.html
see:
http://www.insightcentral.net/encyclope ... lytic.html