Been a long time since I last saw that on my MKR.
Few questions for you
When you say resetting do you mean single or the "quick double tap" to put it in bootloader mode ?
Which version of the IDE ? (1.6.11 being the most stable for my MKR's)
Which version of the boards for the Cortex M0 ? (currently best with 1.6.6)
Are you using USB 3.0 ports ? (If so there are known issues with USB 3.0 try USB 2.0 ports)
Have you tried different USB cables ? (not all micro USB cables are the same, some are power only)
If you are using a USB hub is it a POWERED one ? (Many non powered hubs may not work)
Did you search the forums for "could not find specified port" ? (question has been asked before I am sure)