You've got to check the "Enable FC Main Voltage" check box, otherwise MWOSD reads voltage from its own voltage pin instead of displaying the voltage sent by the flight controller (through MSP protocol). Alternatively you may compile MWOSD with the following option in config.h : "define FC_VOLTAGE_CONFIG".
Connect / Disconnect loop is a long standing issue: Samsung USB cables and a powerfull usb ports do help a bit, but your mileage may vary... On my laptop, the usb port dedicated to the external DVD had a 2A output. The other ports were regular 0.5A ones. The beefy port was less troublesome. Same story with my desktop: the front panel is less reliable than the usb ports that are located in the back of the PC. As a last resort, some are using USB hubs with dedicated power supply.
Hope this helps