Drupal Cron getting stuck

I've just run into the drupal cron problem again  

Cron fails and the error log says

"Attempting
to re-run cron while it is already running."

• delete from variable where name='cron_semaphore';
• clear caches
• run cron from the web page

That seems to do the trick.

Now cron runs OK fom drush or web.