If you are seeing drops (I wouldn't call them errors myself, even if they do get lumped in with them), then it's output buffer congestion on the port, at least in principle. So it's higher level things like flow control, QoS, speed mismatches and traffic patterns I would look at, if it's a problem.
I would tend to look at the RX side counters for cabling and duplex faults: if they're clean, the link is probably OK.