How long are you leaving your phone off before reattempting to reboot? The n900 does need to be left powered off for at least 8 seconds before attempting to reboot.
reflash kernel only, it won't affect apps you have installed. flasher-3.5.exe -F RX-51_2009SE_21.2011.38-1_ PR_COMBINED_MR0_ARM.bin --flash-only=kernel -f -R