New to iPXE, struggling with inconsistent sanboot connections

[NiKiZe]

Hi, and welcome to iPXE,

First of I would like to warn you about using ipxe to boot isos it might work, but it's not optimal.
One thing to know is that iso boot relies on Legacy BIOS INT13 which is only available up to a certain point in the boot process (until something else takes over) For example memtest will work since it relies on BIOS to load and run, however things like linux or windows or other full blown OSes will not be able to run in this manner, since there won't be any ISO available when the real OS kernel takes over.

For memtest there is even memtest.0 which mcb30 (the main iPXE developer) have created that is an NBP, that means it can start memtest, and even exit memtest and return to iPXE with the information about if the test was successful or not, (it is even possible to create a screenshoot from ipxe and get a dump like: http://b800.org/BgsDR )

Sorry for that long rambling.
Now to look at your current issue we see: http://ipxe.org/2d03e13b

on that page there is additional notes about what might have been the cause, and recommended ways to debug/fix the issue.
My hunch tells me that dnsmasq is not always sending filename, (since ipxe is not even trying to fetch it)
Other then the recommendations on the error page ... maybe compare the IP and gw between the screen output when it fails/works to see if everything is as expected, maybe there is a rouge DHCP server on the network.

I don't know much dnsmasq so can't tell if it's anything in your config or not (I'm a firm ISC dhcpd beliver)
Hmm, does 192.168.1.150,192.168.1.240 mean it only gives 240-250 ip's? but the IP in the failing screen is 137?