How long has it been since the board was last calibrated? We have a great web-based utility that will perform a diagnostic test on your board. The test also includes a self-calibration. This could help pin-point any problems with your hardware, as well as calibrating your card for better accuracy.
Online E Series Diagnostic Utility
http://www.ni.com/support/selftest/
Regards,
Justin Britten
Applications Engineer
National Instruments
