First, flash any froyo (2.2) rom.
Then, take out the battery, then connect to the wall charger (not the PC) without the battery. Put the battery back in if you get the android logo.
I use this method to boot my Milestone when its being problomatic. At the moment, its the only way mine will boot. With mine, I'll get the m logo, then it will panic and reboot. It does this for 5 to 10 minutes, then boots up. Once its booted, everything works fine.
This method 'appears' to bypass some of the checks for normal booting. If I try to boot normally, it will bootloop for hours or until the battery runs out.
You need to flash froyo because when there is a problem, 2.2 kernal panics and tries rebooting. In eclair (2.1) when theres a problem, it just locks at the m logo, so this method wont work.
Also, I dont think flashing with sbf_flash will make a diference, since your not getting any bootloader error messages, the flash is apparently good, something else is causing your problem.
Just to check, you are flashing non-service roms, aren't you? Also try taking out your sim card and sd card, just to make sure they aren't causing problems.