randomly getting 403 forbidden error

[rapfan08]

After building my website around prestashop, the site might work for a couple hours/days then all of a sudden i receive a 403 forbidden error saying:You don’t have permission to access / on this server....then after a couple of minutes/hours its up again

well im not sure if its permissions or the index or what…..it’s kinda strange…
there’s nothing about the error in the error log when it occurs