I got a little over ambitious and ordered 4 of the new Pi 3B+ units to replace increasingly older ION boxes. I thought, "New boards are faster with gigabit and that's better!" Turns out the new ones are having quite a few problems with networking for a variety of reasons. I was getting frequent drops of network data and sometimes drops of the network link altogether. This would cause my Pi to soft lock, stutter, poor playback, etc. It was worst on twitch streams using the twitch plugin. Streams would hang, buffer, drop to 1fps, and just disconnect. I thought I had defective Pis and bad PSUs. I have 4 of both so I swapped parts and the issue persisted. I even tried another media distro with similar issues.
If you get network instability on the Pi 3B+ here are a two quick things to try. First, check your power. Even if you're convinced it isn't the problem, replace it as a troubleshooting step. Thin wires and cheap cables can cause a big voltage drop. The Ethernet controller is the first thing to go when the voltage sags. If the problem persists try adding dtparam=eee=off to your config.txt file and reboot. This fixed it for me. I use cheap 8-port gig switches (Trendnet) and apparently the 3B+ doesn't like them. This also is apparently an issue on long Ethernet runs.
Hope this helps others. I spent a lot of time reading, reformatting, testing, and troubleshooting network components. If you have network stability problems this is easy and should probably be high on the list of stuff to try.
If you have network performance issues - especially sending data to the Pi at gig speeds, refer here RPi 3B+ gigabit ethernet bad download speeds. - Raspberry Pi Forums
