Setting cron job for files and folder permissions

Question

Each time a new file is generated at var/cache/mage-tags/* it does not have the correct rights and the customers can't access the online store. The only way to make it back to normal is to log in by SSH > go to root of magento and perform chown -R user:psaserv .

I have tried to remove the prefix from the new generated files but still the error appears from time to time.

The olnly thing that has left me is to set up a cron job that performs each 5 or maybe 10 minutes executing the command chown -R user:psaserv .

So I will be shure that even a new file is generated it will have the right permissions so the store will be available for customers and it won't stop.

I am using plesk as administration is someone familiar how can I set such a cron job?

The main directory of my magento is /var/www/vhosts/website.com/httpdocs/