Could be a heat problem, have a look at the cpu-cooler.
Had the same problem here, cron start >> server shut down no entry in any log.
Again updated with no problems
1 box Mandriva 2007.0 updates from 2.2.7 >> 2.2.9
1 box Mandriva 2007.0 x86_64 also 2.2.7 >> 2.2.9
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