1.7.5 cron use lots memory maybe your sever is set to kill processes that use to much 
look in your error logs and ow cron error log may give you a clue 
also in cpanel cron job set email you on each cron run to see if works right
the server admin will be getting emails on the memory high usage on cron run 
in the email will say if process was killed or not 

Thank you Tammy I will look into these areas you suggest

cool