Strange perodic fluctuations in voltage measurement

Hello there,
I had set up a very simple VI for measuring the pressue via a USB-6009 DAQ unit. I had no problems reading voltage from several pressure transducers until just about a week ago. Some strange periodic fluctuations came from nowhere (see attachment). I tried new power supply, new DAQ unit, rewriting the VI. None of this worked. Could someone help me solve this problem?
Thanks,
yanxiang
Attachments:
fluctuations.png ‏5 KB

I would check around the lab to see if you have any kind of equipment that works at that period.  Like maybe a chamber with a solenoid.
Bill
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