PowerEdge R350 - extremely slow Lifecycle Controller + unable to PXE boot

We got something over 30+ brand new servers poweredge R350 and we are trying them - installing operating system via PXE boot. I have sucessfully installed almost every of them except 6 pieces. I'm using literally the same procedure on each one and it was no problem at all besides these 6 pieces. Mainly, I'm unable to install via PXE boot - I receive IP address from DHCP, it starts to loads the files, but then it get stuck. There is also problem with exteremely slow Lifecycle Controller (when I click I wait 15 seconds for some action even longer, moving cursor on display draws trail of black squares behind) - when I power on server and click on F10, I also wait like 5 minutes to load Lifecycle Controller, normally it should be within 1 minute. I think slowness of Lifecycle Controller and being unable to PXE boot will relate to each other, because this combination appears on all the 6 servers... Nonethless, BIOS itself works smoothly.