I've cleaned the old and used a new paste.
When it boots it works normaly until I power it off.
The fans start normaly.
The screen is completly black no chime and the power led fade in and out.
AS for the OS I don't think is relevant at this boot level but is Leopard with all the updates, sorry can't remember the version.
I think that is somthing related with soldering that expands with the heat and stops making contact.
As for a problem with the sensor, it could be but I think it would't work hours without shuting down.
In the original state it only booted twice in 3 months...
Then a friend told me to try a heat gun.
So we did it very carfuly for keeping the components in place and using a screw driver to apply a small pressure.
First we did it in the CPU and after in the chipset, and now is in this state that is far better, it boots every day as long as is cold!