The larger SP-540V with ethernet had it's own issues but the 300V's as they age all suffer degradation around USB connectivity. I've often thought of trying to track the problem down but in the end just worked around the problem.
I've managed to keep 300V's going but the trick seems to be providing the most optimal USB connection - shortest length cable possible (ideally one of the early USB cables with lots of wire and typically shielding and ferrites), computer as close to the printer as possible, maybe check the USB on the computer (adding a card may help, not all are created equal), avoid cold like the plague (one of my SP-300V's if I open an outside door next to it in winter within 30 seconds it's lost the connection so I've placed insulation on the side panel around the USB socket) and the room's maintained at 22C year round.
I used to run my SP-300V's over some distance from the computer with powered USB cables from the computer to the printers but in the end placed a micro PC (no keyboard, monitor or mouse) right next to the SP-300V with a 250mm USB cable and remote onto the computers via RDP over the ethernet. This has kept my SP-300V's operational long after most of the others in my area have gone off to the scrap heap and they still crank out great work most days.
Good luck