Usb voltage/current problem with pulsesensor

Hello; I have a project right now using pulse sensor from www.pulsesensor.com. I am using 2011 Macbook Air + Arduino Uno + Pulse Sensor (using 3.3v ) + Force Sensitive Resistor. The installation place is far. When I went there sensor was not working (which turn out that sensor is not usable anymore) so I changed it with the new one but it was still not working. Then I unplugged the usb cable(which connected to arduino) from that macbook and plugged in to another macbook then It started to work and I could read the pulse clearly. After I plugged it to the previous mac again and it was not working still with that computer (Reading was not accurate). I tried it couple of times to be sure. Then I explained the problem to the exhibitors as it might be a usb port problem. So I took that mac with me to change it with new one or take it to the a/s center. But when I tried it in my home now it seems that it is working fine now. :~ What would be a reason for that? Is it a loss of voltage/current which is happening time to time? What should I do to fix it? Thank you.

It could be a USB cable with a worn connector on it. I had this happen just recently. Try using a different cable.