Just after I posted the last message I saw that the problem was going
on. I used the Recovery CDs to reinstall but it was the same. At last
I found a link to the Compuserve Toshiba Forum, where there were lots
of similar cases. This is a problem with some Satellite 1800 and 1805
models related to overheating. Most people got it solved by updating
the BIOS. Mine is OK, too. In case someone has the same problem and
finds this, he should do that.