A duplex mismatch is the most probable cause of the issues described, where the network connection experiences slow data transfers and intermittent lockups. This occurs when one device in a network connection operates in full-duplex mode and the other in half-duplex, leading to collisions and performance issues. In the scenario provided, the small switch taken from storage may not have been configured to match the duplex settings of the existing network, resulting in these data transfer issues. Other options like reversed TX/RX pinouts, cable crosstalk, or a failed transceiver would typically result in complete connection failures or consistent degradation, not intermittent issues as described.