usb ports affecting sensor readings

hey I don't know if I should have posted this in the Arduino section but it seems a little too specific to my actually project so here we go.

Basically i'm doing a project involving sonars reading pwm and I have found the erratic readings I sometimes get to be caused by which usb port i use. in my first port readings will stabilise when and object is static but will fluctuate wildly when it is moved and then after some time stabilise again. I found this problem was solved by using my second port until a few days ago. similarly I found a new micro b to usb lead solved the problem.

I've found nothing on this by goggling , i'm running an Arduino leonardo off of a mac book pro with the 10.7.5 software.