Jeff,
The sampling rates are too slow to reveal any significant information about the interference. If the interference is power line frequency for example, the sampling rate would need to be > 120 Hz to have any chance of providing a clue about the interference. 500 Hz or 1000 Hz would be better.
Interestingly, the interference on both channels is highly correlated (using my uncalibrated eye-brain correlator). There are relatively flat spots at 75-85 on both channels as well as several other visible "features." While this does not tell us anything about the interference source, it does suggest that the same interference is happening to both channels.
Can you try creating a current source using a 1.5 V battery and a resistor (possibly the resistor in the 9203) with very short leads? This would have no ground loops and limited opportunity to pick up noise from other sources.
Lynn
