I did call and found something interesting. Apparently with the older (8.0) driver what was supplied with the first USB-6008, when it was set to limits of 0-5.0V output, it would actually only go from 0-4.99V. When 5.00 V was requested, the output went to zero. It was not clear initially that this applied to all units. Installing version 8.3 of the drivers appears to have solved the problem.