This is very interesting.
I also use Powatool, and screen calibration works for me just fine. (I'm on the 3.2010.02-8 firmware.)

The version of Powatool I had was 0.1 - no spontaneous reboots yet again.

Today, i took my n900 to a Nokia service center.
It will take about 3 to 30 days before i will know what's the problem with my phone...
I hope isn't a hardware problem.
In your oppinion do you think this problem is a hardware or software?
The problem:
I bought a n900, with restart loop problem.
Ive installed the first global firmware and it doesen't recognise the sim card.
I've did a OTA update with the second global FW, and now i cant use the internal mass memory...but the simcard is recognised.
Finally, i did the final update OTA and the phone, recognise my sim card, i can use the mass memory, but i win a reboot loop...
Flashing with the same FW, or from another region, like us, uk, or Midle east version, i get similar problems, but not in that order.
Yesterday, after ive installed the latest FW, the phone was perfect, but 2h later, i got reboot loop, without installing anything.
Thank you for your time.

C'mon guys, anyone?

1) Reflash your eMMC (also known as the "vanilla" image)
2) Reflash your N900 with the most current global firmware (3.2010.02-08, or something like that.)

If this doesn't help, there is some more serious trouble.

Indeed,it was a bigger problem. Ive returnd the device to nokia. I think it was a hardware problem.