fastboot stuck on sending 'boot'
Alternatively, because I'm guessing you're in a process to flash a new Custom ROM and you want to flash manually using fastboot, you can look for a compatible custom recovery, download it and place it in the computer you're using. On fastboot, if bootloader is unlocked, as it probably might be since you're already on custom ROM, write these commands

fastboot boot [path-to-custom-recovery]


It will boot up on a custom recovery that you downloaded


Select Wipe
Advanced Wipe
Check system, dalvik, data, cache
Swipe to wipe
Back to install
Locate the zip and install it or send it from computer using adb sideload