Can you test the game with Flow Control ON or Flow Control OFF?

I'm not using a switch - my Edgerouter has enough ports. It does not however support flow control.

I should point out, it didn't do this before; I had months of solid gameplay. Now, I get maybe 10 minutes before having to restart it.
I don't think it's the Link, either - falling back to old builds hasn't helped, nor has switching to the stable build of Steam. I've also disabled hardware decoding/hardware encoding at various intervals, to no avail. However my Steam games play GREAT, just not on the Link. So something must be going on.

I do have VNC running, I wonder if that is related? EDIT: nope, disabling VNC didn't help.

gedit /etc/sysctl.conf

### IMPROVE SYSTEM MEMORY MANAGEMENT ###

# Increase size of file handles and inode cache
fs.file-max = 2097152

# Do less swapping
vm.swappiness = 10
vm.dirty_ratio = 60
vm.dirty_background_ratio = 2

### GENERAL NETWORK SECURITY OPTIONS ###

# Number of times SYNACKs for passive TCP connection.
net.ipv4.tcp_synack_retries = 2

# Allowed local port range
net.ipv4.ip_local_port_range = 2000 65535

# Protect Against TCP Time-Wait
net.ipv4.tcp_rfc1337 = 1

# Decrease the time default value for tcp_fin_timeout connection
net.ipv4.tcp_fin_timeout = 15

# Decrease the time default value for connections to keep alive
net.ipv4.tcp_keepalive_time = 300
net.ipv4.tcp_keepalive_probes = 5
net.ipv4.tcp_keepalive_intvl = 15

### TUNING NETWORK PERFORMANCE ###

# Default Socket Receive Buffer
net.core.rmem_default = 31457280

# Maximum Socket Receive Buffer
net.core.rmem_max = 12582912

# Default Socket Send Buffer
net.core.wmem_default = 31457280

# Maximum Socket Send Buffer
net.core.wmem_max = 12582912

# Increase number of incoming connections
net.core.somaxconn = 4096

# Increase number of incoming connections backlog
net.core.netdev_max_backlog = 65536

# Increase the maximum amount of option memory buffers
net.core.optmem_max = 25165824

# Increase the maximum total buffer-space allocatable
# This is measured in units of pages (4096 bytes)
net.ipv4.tcp_mem = 65536 131072 262144
net.ipv4.udp_mem = 65536 131072 262144

# Increase the read-buffer space allocatable
net.ipv4.tcp_rmem = 8192 87380 16777216
net.ipv4.udp_rmem_min = 16384

# Increase the write-buffer-space allocatable
net.ipv4.tcp_wmem = 8192 65536 16777216
net.ipv4.udp_wmem_min = 16384

# Increase the tcp-time-wait buckets pool size to prevent simple DOS attacks
net.ipv4.tcp_max_tw_buckets = 1440000
net.ipv4.tcp_tw_recycle = 1
net.ipv4.tcp_tw_reuse = 1

My /etc/sysctl.conf was unmodified previously, and it worked for months - why would it suddenly require additional tuning?

Either way, I'll use this as a base and see what happens.

Edit:
Nope, problem returned immediately. Multiple attempts.

Better try to install a gigabyte switch at least you will know for sure that is not something with your router.

I had in the past some weird problems with my router when I was streaming movies from my NAS to TV like scrambling and freezing some times.Now my router just handles the traffic through the Internet and I dont overload it with local network streaming etc,My switches handle that.
I have like 12 devices on my home network and all are using my 2 gigabyte switches instead of router ports and never had an issue so far.