A disk read error occurred (under load)

[robinsmidsrod]

You should really upgrade to iPXE and use HTTP intead of TFTP. HTTP has much better connection tracking (and is completely rewritten in iPXE) and is much less likely to timeout because of resource starvation. HTTP servers are also much better at distributing load with e.g. a reverse proxy (like haproxy or squid) in front of it. As long as network bandwidth is not your bottleneck you should try out this approach.