Graphics Corruption

[NiKiZe]

Short answer: don't use Grub, I'm sure there is better solutions, but we know to little about your use-case to give a good answer on what those better solutions are ... but why are you starting Grub in the first place?

It might help if you could upload an image of what you see elsewhere and add a link to it here.
My guess is that it is either an Grub issue or bios issue.
One workaround might be to enable the console command in your iPXE build, and use that to change resolution.