Well, quite interesting what happens. I'll try to expain:

I have issued the R&D command as I copy below:
ubuntu@ubuntu:~$ sudo flasher-3.5 --enable-rd-mode --set-rd-flags=no-omap-wd
flasher v2.5.2 (Oct 21 2009)
Suitable USB device not found, waiting.
USB device found found at bus 002, device address 008.
Found device RX-51, hardware revision 2101
NOLO version 1.4.14
Version of 'sw-release': RX-51_2009SE_21.2011.38-1_PR_MR0
The device is now in R&D mode

Now I have started the device in R&D mode (a tool icon appears in the nokia logo) with the keyboard opened. Bootmenu started correctly, but after a while, the phone powered off as before.

Then I returned the phone to the production mode:
ubuntu@ubuntu:~$ sudo flasher-3.5 --disable-rd-mode
flasher v2.5.2 (Oct 21 2009)
Suitable USB device not found, waiting.
USB device found found at bus 002, device address 010.
Found device RX-51, hardware revision 2101
NOLO version 1.4.14
Version of 'sw-release': RX-51_2009SE_21.2011.38-1_PR_MR0
The device is now in production mode

Trying to do something new, I restarted the N900 (in production mode) and started the second option of the menu. A commandline appeared and I was wainting for the power-off. But there was no power off!!! The commandline was there and I can issue commands without problems.

I restarted the phone again to select the USB mass storage mode and now the phone does not power-off itself!!! The problem has gone.

Do you think that there was a problem with the watchdogs? What could be happening? Something changed after activating and deactivating the R&D mode?

Thanks.