I fix few boot-loop units… with NetInstall and Wireshark, check my HowTo http://forum.mikrotik.com/t/mikrotik-basebox-keeps-beeping-rebooting/143638/2
Remember that some version of NetInstall.exe not work properly and try use other, e.g. http://forum.mikrotik.com/t/why-last-netinstall-cant-boot-any-x86-devices-deep-dive/149846/1