My method to come into fastboot-mode while having a bootloop:
1. when in bootloop press all buttons (power, VolUp, VolDown) and keep pressed
2. when screen goes off, leave the VolUp button alone, keep VolDown and Power pressed.
3. when phone vibrates, leave power alone and keep VolDown pressed
4. the bl-unlock-warning shows up and after a few seconds you should be in fastboot-mode
5. now you can leave VolDown alone
Maybe you need some tries, cause its a question of timing, but for me it works.
I was atheist until I realised I am GodRazr I -> infiniROM + AOSP-ThemeMod
Xoom 2 -> root + safestrap
Defy green -> CM7 maniac