A stable file transfer should look similar to a central value, as shown on the left-hand side of the following graph. An unstable file transfer can look like the graph on the right, where the throughput graph is jumping up and down:

You can also see the throughput in the I/O graph. The main point to note is that the I/O graph shows you the throughput of all the traffic in the trace file, in both directions, while the TCP stream throughput graph shows only the throughput from one TCP stream in one direction, based on the selected packet. If you filter the I/O graph so that it is looking at the same traffic as the throughput graph, you'll see the same values for bytes/second.