Problems with DISM and system restore

I'm running Windows 8.1 Pro on a Dell Inspiron 660.

I've been getting the occasional BSOD. Ran DISM /Online /Cleanup-Image /RestoreHealth and got the message: "Error 0x800f0906...The source files could not be downloaded..."

I've then tried to do a system restore. Eventually got the message "Restore did not complete succesfully....System restore failed to extract the file C:Windows from the restore point... Error 0x80070005". Tried different restore points, running it in safe mode, shutting down Avast AV system, but still ended up with the same result.

Help!