I found that I had to connect signal common to earth ground through a 10K ohm resistor or I got "motorboating" with no gas flow. Offset still around 0.1 volts which is OK.
My boss suggested I tie the analog signal common to the bipolar power supply common, which solved the problem. I had forgotten that in bipolar power supplies the ground can "float" one polarity or the other with respect to signal ground unless they are joined. The clue was that the offset I was seeing varied from near zero when the analog value was zero to more than .25V when the analog was at 5V full scale.