Depending how long ago you bought it, it's probably best that you open a ticket with tech support. If your unit is still in warranty, then I would definitely check with support and see if they can't help you troubleshoot the situation, before you go wiping the drives.
How old is your linkstation? When did you purchase it? Was this something you purchased new? Or is this something you received from someone who had used it before?
If you're going to wipe the drives and start over, read the articles from this post below first, before you do anything. They cover everything needed to get the drives set up again, after they are wiped.
https://forums.buffalotech.com/index.php?topic=30419.0TFTP is a windows app you run to host the boot files. When I do TFTP, I run it from from a second PC. It just acts as a file server to pass the boot files to the linkstation. That's all it does.
The other way is to not use TFTP, it requires linux drive formatting from a linux pc, and in my experience, it has never worked.
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Fixed wiring? I wonder honestly- if this is part of the problem. Are you talking about internal wiring, behind walls? If so- that kind of wiring may very well not be up to 1 gigabit standards, and it may be causing some flaky connections too.
The best way to test temporarily, is to put all the devices (one at a time) directly on the same switch in the same room with known good patch cables, and test them that way. I wonder if you still get these errors and drop offs in speed with the fixed wiring eliminated.
Mind you- the max transfer speed you will get on a good day with SMB2- will be 30MB/sec for one connection. And expect less if multiple devices access it- at the same time.
Just because the NAS has gigabit capability, doesn't mean it will hit the maximum throughput. Even at 20MB/sec, it is utilizing some part of the gigabit- at greater than 100mbit speeds. It will never hit a good 100 MB/sec like transfers between PCs, due to the hardware limitations of the linkstation. Lack of speed and other reasons- are why I opted to later upgrade to a Terastation. Linkstations are good starter units anyway.