Backup To Network Drive Runs Out Of RAM

I have been using Win 7 Professional, Backup and Restore sucessfully for almost a year where I have been backing up my computer (250Gbyte Sys Image and 150Gbyte User Folder) to a network dual RAID 2TB each hardrives. A few months ago, these backups failed to complete sucessfully, causing the computer to lock up. At first, I thought it was becuase I left the computer running for too many days but it seemed to lock up every Sun morning which is when I discovered that it coincided with my scheduled backup time. What happens is during backup, the RAM memory slowly increases until it gets into the mid 90%, the computer locks up and displays an error message, not enough memory.I have reduced my backup size from 150Gbyte to 50GByte and I still get the same result, I have completely started over with a new backup setup, selecting the network drive etc. No luck there.