It is "RAM" issue, not storage space issue. There are too many applications running in the background which are eating up your phone memory. N73ME when start on will have about 17 to 20 MB of ram avilable. Applictions start after that will eat those MB. For example, camera app after used, it never completely close. You have to use app like Mobile tracker to have a look how your phone manage its ram. So if you don't want to manage your own phone applications, it is not advised, as for me, not to leave you phone on forever. Turn it off at least once a day, to get it fresh start again.
C6-00 with 8GB MicroSD