USB-to-Serial adaptors will almost never be assigned the COM1-COM4 ports; those are reserved for physical ports as they have specific addresses and IRQ3 and IRQ4 reserved for them. They are generally assigned a higher COM port, 8-12. One reason COM ports change is if a program has the COM port open that port will not be assigned. So if your USB port is loose and the USB-to-Serial adaptor has intermittent connection, and a terminal program has the port open, then the same adaptor will be assigned a new COM port.